Etching solution for etching porous silicon, etching method using the etching solution and method of preparing semiconductor member using the etching solution

ABSTRACT

A method for preparing a semiconductor member comprises: 
     forming a substrate having a non-porous silicon monocrystalline layer and a porous silicon layer; 
     bonding another substrate having a surface made of an insulating material to the surface of the monocrystalline layer; and 
     etching to remove the porous silicon layer by immersing in an etching solution.

This application is a division of application Ser. No. 08/472,270, filedJun. 7, 1995, now pending, which in turn is a division of applicationSer. No. 07/835,381, filed Feb. 14, 1992, now U.S. Pat. No. 5,767,020.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an etching solution for porous silicon, anetching method using the etching solution and a method of producing asemiconductor member using the etching solution. Particularly, thisinvention relates to a method of producing a semiconductor member whichis suitable for separation of dielectric materials or electronicdevices, integrated circuits prepared on a monocrystalline semiconductorlayer on an insulating material.

2. Related Background Art

Formation of a monocrystalline Si semiconductor layer on an insulatingmaterial has been widely known as the silicon on insulator (SOI)technology, and since a large number of advantages which cannot bereached by bulk Si substrates for preparation of conventional Siintegrated circuits are possessed by the device utilizing the SOIstructure, so many researches have been done. More specifically, byutilizing the SOI structure, the following advantages can be obtained:

1. Dielectric isolation can be easily done to enable high degree ofintegration;

2. Radiation hardness is excellent;

3. Stray capacity is reduced to attain high speed;

4. Well formation step can be omitted;

5. Latch-up can be prevented;

6. Fully depleted field effect transistor can be made by thin filmformation.

In order to realize the many advantages in device characteristics asmentioned above, studies have been made about the method for forming theSOI structure for these some 10 years. The contents are summarized in,for example, the literature as mentioned below:

Special Issue: “Single-crystal silicon on non-single-crystalinsulators”; edited by G. W. Cullen, Journal of Crystal Growth, Volume63, No. 3, pp. 429-590 (1983).

Also, it has been known for a long time to form the SOS (silicon onsapphire) structure by heteroepitaxy of Si on a monocrystalline sapphiresubstrate by CVD (chemical vapor deposition) method. This was successfulto some extent as the most mature SOI technique, but for such reasons asa large amount of crystal defects because of lattice mismatching at theinterface between the Si layer and the sapphire substrate, introductionof aluminum from the sapphire substrate into the Si layer, and above allthe high cost of the substrate and delay in enlargement of the substratewafer size, it is obstructed from being widely applied. In recent years,attempts to realize the SOI structure without use of a sapphiresubstrate have been done. Such attempts may be broadly classified intothe two shown below:

(1) After surface oxidation of an Si monocrystalline substrate, a windowis formed to have the Si substrate partially exposed, and epitaxialgrowth is proceeded in the lateral direction with that exposed portionas the seed to form an Si monocrystalline layer on SiO₂. (In this case,deposition of Si layer on SiO₂ is accompanied).

(2) By use of an Si monocrystalline substrate itself as an active layer,SiO₂ is formed therebeneath. (This method is accompanied with nodeposition of Si layer).

As the means for realizing the above (1), there have been known themethod in which a monocrystalline Si layer is formed directly to lateralepitaxial growth by CVD, the method in which amorphous Si is depositedand subjected to solid phase lateral epitaxial growth by heat treatment,the method in which an amorphous or polycrystalline Si layer isirradiated convergently with an energy beam such as electron beam, laserbeam, etc. and a monocrystalline layer is grown on SiO₂ by melting andrecrystallization, and the method in which a melting region is scannedin a zone fashion by a rod-shaped heater (Zone meltingrecrystallization). These methods have both advantages anddisadvantages, they still have many problems with respect tocontrollability, productivity, uniformity and quality, and none of themhave been industrially applied to date. For example, the CVD methodrequires sacrifice-oxidation in flat thin film formation, while thecrystallinity is poor in the solid phase growth method. On the otherhand, in the beam annealing method, problems are involved incontrollability such as treatment time by converged beam scanning, themanner of overlapping of beams, focus adjustment, etc. Among these, theZone Melting Recrystallization method is the most mature, and arelatively larger scale integrated circuit has been trially made, butstill a large number of crystal defects such as point defects, linedefects, plane defects (sub-boundary), etc. remain, and no device drivenby minority carriers has been prepared.

Concerning the method using no Si substrate as the seed for epitaxialgrowth which is the above method (2), for example, the following methodsmay be included.

1. An oxide film is formed on an Si monocrystalline substrate withV-grooves as anisotropically etched on the surface, a polycrystalline Silayer is deposited on the oxide film thick to the extent as the Sisubstrate, and thereafter by polishing from the back surface of the Sisubstrate, Si monocrystalline regions dielectrically separated bysurrounding with the V-grooves on the thick polycrystalline Si layer areformed. In this method, although crystallinity is good, there areproblems with respect to controllability and productivity in the step ofdepositing the polycrystalline Si thick as some hundred microns and thestep in which the monocrystalline Si substrate is polished from the backsurface to leave only the Si active layer as separated.

2. This is the method called SIMOX (Separation by ion-implanted oxygen)in which an SiO₂ layer is formed by ion implantation of oxygen into anSi monocrystalline substrate, which is one of the most mature methodsbecause of good matching with the Si-IC (Integrated Circuit) process.However, for formation of the SiO₂ layer, 10¹⁸ ions/cm² or more ofoxygen ions are required to be implanted, and the implantation time isvery long to be not high in productivity, and also the wafer cost ishigh. Further, many crystal defects remain, and from an industrial pointof view, no sufficient level of quality capable of preparing a devicedriven by minority carriers have been attained.

3. This is the method to form an SOI structure by dielectric isolationaccording to oxidation of porous Si. This is a method in which an N-typeSi layer is formed on the surface of a P-type Si monocrystallinesubstrate in shape of islands by way of proton ion implantation (Imai etal., J. Crystal Growth, Vol. 63, 547 (1983)), or by epitaxial growth andpatterning; only the P-type Si substrate is made porous by anodizationin HF solution so as to surround the Si islands from the surface; andthen the N-type Si islands are dielectrically isolated by acceleratedoxidation. In this method, the separated Si region is determined beforethe device steps, whereby there is the problem that the degree offreedom in drive and circuit design may be limited in some cases.

A light-transmissive substrate is important for forming a contact sensorserving as a light-receiving device and a projection-type liquid crystalimage display. A high-quality driving device is required for furtherincreasing the density, resolution and definition of the pixels (pictureelement) of such a sensor or display. It is consequently necessary toproduce a device to be provided on a light-transmissive substrate byusing a monocrystalline layer having excellent crystallinity.

However, if an Si layer is deposited on a light-transmissive substratesuch as glass substrate, etc., the Si layer is generally an amorphouslayer or, at best, a polycrystalline layer because the Si layer reflectsthe disorder of the crystal structure of the substrate, and nohigh-quality device can thus be formed by using the Si layer. This isbecause the substrate has an amorphous crystal structure, and thus amonocrystalline layer of high quality cannot be easily obtained bysimply depositing the Si layer. It is therefore difficult to produce adriving device having properties sufficient for the present demands orfuture demands because the crystal structure of an amorphous Si orpolycrystalline Si has many defects.

Further, any one of the methods using an Si monocrystalline substrate isunsuitable for obtaining a good monocrystalline film on alight-transmissive substrate.

Takao Yonehara, one of the inventors, previously proposed a method offorming a semiconductor substrate which is capable of solving the aboveproblems in Japanese Patent Application No. 2-206548.

The method of forming a semiconductor substrate disclosed in PatentApplication No. 2-206548 comprises forming a substrate having anon-porous semiconductor monocrystalline layer and a poroussemiconductor layer, bonding another substrate having an insulatingmaterial surface to the surface of the monocrystalline layer, andremoving the porous semiconductor layer by etching.

This invention has been achieved for improving the invention disclosedin Patent Application No. 2-206548 previously proposed.

The method of forming a semiconductor substrate disclosed in PatentApplication No. 2-206548 comprises the step of removing porous Si byselective etching.

Porous Si is described below.

Porous Si was discovered in the course of research on electrolyticpolishing of a semiconductor which was conducted by Uhlir et al, in 1956(A. Uhlir, Bell Syst. Tech. J., Vol. 35, pp 333 (1956)).

Unagami et al. investigated dissolving reaction of Si during anodizationand reported that the anodic reaction of Si in a HF solution requirespositive holes, and that the reaction is expressed as follows (T.Unagami, J. Electrochem. Soc., Vol. 127, pp 476 (1980)):

Si+2HF+(2−n)e⁺→SiF₂+2H⁺+ne⁻  (1)

SiF₂+2HF→SiF₄+H₂  (2)

SiF₄+2HF→H₂SiF₆  (3)

or

Si+4HF+(4−λ)e⁺→SiF₄+4H⁺+λe⁻  (4)

SiF₄+2HF→H₂SiF₆  (5)

wherein e⁺ and e⁻ respectively denote a positive hole and an electron,and n and λ each denotes the number Of positive holes required fordissolving one silicon atom. Porous Si can be formed when the condition,n>2 or λ>4, is satisfied.

It is therefore found that positive holes are required for formingporous Si, and that P-type Si can be more easily made porous than N-typeSi. However, it is also known that N-type Si can be made porous if holesare implanted thereto (R. P. Holmstrom and J. Y. Chi, Appl. Phys. Lett.,Vol. 42, 386 (1983)).

The density of the porous Si layer can be changed to the range of 1.1 to0.6 g/cm³ by changing the concentration of the HF solution from 50 to20%, as compared with the density of 2.33 g/cm³ of monocrystalline Si.The porous Si layer has pores having an average size of about 600 Åwhich was measured by observation by a transmission electron microscope.Although the porous Si layer has a density which is half or less thanthat of monocrystalline Si, monocrystallinity is maintained, and amonocrystalline Si layer can be formed on the porous layer by epitaxialgrowth.

Although the volume of an Si monocrystal is generally increased by 2.2times by oxidation, the increase in volume can be suppressed bycontrolling the density of the porous Si so that the occurrence ofcurvature of a substrate or the occurrence of a crack in amonocrystalline layer remained on the surface can be avoided during theoxidation process. The volume ratio R of monocrystalline Si to porous Siafter oxidation can be expressed as follows:

R2.2×(A/2.33)  (6)

wherein A denotes the density of porous Si. If R=1, i.e., there is noincrease in volume after oxidation, A=1.06 (g/cm³). Namely, if thedensity of the porous Si layer is 1.06, an increase in volume, which iscaused by oxidation, can be suppressed.

It can be said that at present, porous Si is subjected as such directlyto subsequent steps (epitaxial growth and oxidation) after productingit, and the porous Si itself is not processed. This is because theporous Si cannot be easily processed or removed with goodcontrollability. Namely, it has been not reported yet that porous Si isetched with good controllability.

In addition, P generally shown by the following equation is referred asporosity:

P=(2.33−A)/2.33  (7)

When the value of porosity is adjusted to 30 to 55% during anodization,the properties of oxidized porous Si can be equalized to those of amonocrystalline Si oxide film. The porosity is expressed as follows:

P=(m1−m2)/(m1−m3)  (8)

or

P=(m1−m2)/ρAt  (9)

wherein

m1: total weight before anodization

m2: total weight after anodization

m3: total weight after removal of porous Si

ρ: density of monocrystalline Si

A: area of porous region

t: thickness of porous Si

However, the area of the porous region cannot be accurately calculatedin many cases. In this case, although the equation (8) is effective, theporous Si must be etched for measuring the value of m3.

In addition, during epitaxial growth on the porous Si, the porous Si iscapable of relieving distortion produced during heteroepitaxial growthand suppressing the occurrence of defects. However, in this case, sinceit is clear that the porosity is a very important parameter, measurementof the porosity is necessary and indispensable.

Known methods of etching porous Si are the following methods (1) and(2):

(1) The method of etching porous Si with an aqueous NaOH solution (G.Bonchil, R. Herino, K. Barla, and J. C. Pfister, J. Electrochem. Soc.,Vol. 130, No. 7, 1611 (1983)).

(2) The method of etching porous Si with an etching solution which iscapable of etching non-porous Si.

In the above method (2), a fluoronitric acid-type etching solution isgenerally used, and etching of Si proceeds as follows:

Si+2O→SiO₂  (10)

SiO₂+4HF→SiF₄+H₂O  (11)

As shown by the above reaction formulas, Si is oxidized to SiO₂, and theSiO₂ produced is etched with hydrofluoric acid.

Examples of etching solutions for non-porous Si include the abovefluoronitric acid-type etching solution as well as ethylenediamine-type,KOH-type and hydrazine-type etching solutions and the like.

In this invention, it is necessary in selective etching of porous Si toselect an etching solution which is capable of etching porous Si, otherthan the above etching solutions for non-porous Si. The porous Si isgenerally selectively etched by the above method (1) which uses anaqueous NaOH solution as an etching solution.

As described above, both porous and non-porous Si are etched with thefluoronitric acid-type etching solution.

On the other hand, in the conventional method of selectively etchingporous Si with an aqueous NaOH solution, Na ions are inevitably adsorbedon the etched surface. Since the Na ions cause impurity contamination,are movable and have adverse effects such as the formation of ainterfacial states, the ions must not be introduced into thesemiconductor process.

SUMMARY OF THE INVENTION

An object of this invention is to provide an etching solution whichefficiently, uniformly, selectively and chemically etches porous Siwithout affecting the semiconductor process and etching non-porous Si.

Another object of this invention is to provide a method of preparing asemiconductor member using an etching solution for uniformly andselectively etching porous Si.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic views explaining an etching step using anetching solution of this invention;

FIGS. 2A and 2B are schematic views explaining an etching step using anetching solution of this invention;

FIGS. 3A to 3C are schematic views explaining an etching step using anetching solution of this invention;

FIGS. 4A to 4C are schematic views explaining an etching step using anetching solution of this invention;

FIGS. 5A to 5D are schematic views explaining an etching step using anetching solution of this invention;

FIGS. 6A to 6H are graphs showing the etching properties of porous andnon-porous Si when etching solutions of this invention are respectivelyused;

FIGS. 7A to 7H are graphs showing the relations between the etchedthickness (etching depth) of porous Si and etching time when etchingsolutions of this invention are respectively used;

FIGS. 8A to 8C are schematic views explaining a process for preparing asemiconductor member of this invention;

FIGS. 9A to 9D are schematic views explaining a process for preparing asemiconductor member of this invention;

FIGS. 10A to 10C are schematic views explaining a process for preparinga semiconductor member of this invention;

FIGS. 11A to 11D are schematic views explaining a process for preparinga semiconductor member of this invention;

FIGS. 12A to 12C are schematic views explaining a process for preparinga semiconductor member of this invention;

FIGS. 13A to 13C are schematic views explaining a process for preparinga semiconductor member of this invention; and

FIGS. 14A to 14D are schematic views explaining a process for preparinga semiconductor member of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to one aspect of this invention, this invention provides achemical etching solution for etching porous silicon.

A first embodiment of the chemical etching solution of this invention ishydrofluoric acid.

A second embodiment of the chemical etching solution is a mixturecontaining hydrofluoric acid and an alcohol.

A third embodiment of the chemical etching solution is a mixturecontaining hydrofluoric acid and hydrogen peroxide.

A fourth embodiment of the chemical etching solution is a mixturecontaining hydrofluoric acid, an alcohol and hydrogen peroxide.

A fifth embodiment of the chemical etching solution is bufferedhydrofluoric acid.

A sixth embodiment of the chemical etching solution is a mixturecontaining buffered hydrofluoric acid and an alcohol.

A seventh embodiment of the chemical etching solution is a mixturecontaining buffered hydrofluoric acid and hydrogen peroxide.

A eighth embodiment of the chemical etching solution is a mixture ofbuffered hydrofluoric acid, an alcohol and hydrogen peroxide.

The etching method of this invention comprises selectively etchingporous silicon using the etching solution of this invention.

According to another aspect of this invention, this invention provides amethod of preparing a semiconductor member.

A first embodiment of the method of preparing a semiconductor member ofthis invention comprises forming a substrate having a non-porousmonocrystalline silicon layer and a porous silicon layer, bondinganother substrate having a surface made of insulating material to thesurface of the monocrystalline layer, and etching the porous siliconlayer by immersing it in hydrofluoric acid.

A second embodiment of the method of preparing a semiconductor member ofthis invention uses each of the second to eighth forms of the etchingsolution of this invention in place of hydrofluoric acid used as anetching solution in the first form of the method of preparing asemiconductor member of this invention.

A third embodiment of the method of preparing a semiconductor member ofthis invention comprises the steps of making a silicon substrate porous,forming a non-porous monocrystalline silicon layer on the siliconsubstrate made porous, bonding a light-transmissive glass substrate tothe surface of the non-porous monocrystalline silicon layer, andselectively etching porous silicon so as to remove porous silicon bychemical etching using an etching solution of this invention byimmersing the silicon substrate made porous therein.

A fourth embodiment of the method of preparing a semiconductor member ofthis invention comprises the steps of making a silicon substrate porous,forming a non-porous monocrystalline silicon layer on the siliconsubstrate made porous, bonding another silicon substrate having aninsulating layer on the surface thereof to the surface of the non-porousmonocrystalline silicon layer, and selectively etching porous silicon soas to remove porous silicon by chemical etching using an etchingsolution of this invention by immersing the silicon substrate madeporous therein.

A fifth embodiment of the method of preparing a semiconductor member ofthis invention comprises the steps of making a silicon substrate porous,forming a non-porous monocrystalline silicon layer on the siliconsubstrate made porous, forming an oxide layer on the surface of thenon-porous monocrystalline silicon layer, bonding a light-transmissivesubstrate to the surface of the oxide layer and selectively etching thesilicon substrate made porous to remove it by chemical etching using anetching solution of this invention by immersing the silicon substratemade porous therein.

A sixth embodiment of the method of preparing a semiconductor member ofthis invention comprises the steps of making a silicon substrate porous,forming a non-porous monocrystalline silicon layer on the siliconsubstrate made porous, forming an oxide layer on the surface of thenon-porous monocrystalline silicon layer, bonding another siliconsubstrate having an insulating layer on the surface thereof to the oxidelayer formed on the non-porous monocrystalline silicon layer, andselectively etching the silicon substrate made porous to remove it bychemical etching using an etching solution of this invention byimmersing the silicon substrate made porous therein.

In each of the above embodiments according to the method of preparing asemiconductor member of the present invention, the etching step may becarried out with coating the surfaces other than the surface of thesilicon layer made porous with a protecting material before etching.

The etching solution for porous Si of this invention is capable ofuniformly and efficiently etching porous Si without the danger ofcontaminating the semiconductor process.

The etching method of this invention can be applied to usualsemiconductor processes and is capable of selectively etching, with highaccuracy, the porous Si provided on the same substrate providednon-porous Si to remove the porous Si because a chemical etchingsolution which does not etch non-porous Si is used.

The method of preparing a semiconductor member of this invention isexcellent in productivity, uniformity, controllability and economy forforming a crystalline Si layer having excellent crystallinity equal tothat of a monocrystalline wafer on insulating substrates such aslight-transmissive insulating substrates represented by a glasssubstrate.

The method of preparing a semiconductor member of this invention iscapable of realizing the advantages of conventional SOI devices and canbe applied to various fields.

The method of preparing a semiconductor member of this invention canalso be used in place of the expensive SOS or SIMOX used for producing alarge-scale integrated circuit having the SOI structure.

In addition, the method of preparing a semiconductor member of thisinvention comprises the steps of chemically removing the lower portionof a monocrystalline Si substrate of high quality used as a startingmaterial, with leaving only a monocrystalline layer on the surfacethereof, and bonding the substrate to an insulating layer, and thusenables many treatments to be performed for a short time and hasexcellent productivity and economy.

Further, the method of preparing a semiconductor member of thisinvention can use a chemical etching solution which has a bad effect onthe semiconductor process in etching of porous Si and exhibits anetching selection ratio of a five digits value or more of porous Si tonon-porous Si and excellent controllability and productivity.

I. A description will now be given of the etching solution in accordancewith the present invention.

I-(1) A description will be made first as to the case where hydrofluoricacid is used as the electroless wet chemical etching solution for porousSi, with specific reference to FIG. 7A.

FIG. 7A shows the etching time dependency of etched thickness of porousSi when the latter is etched by being immersed in hydrofluoric acid. Theporous Si was formed by anodizing a monocrystalline Si. The conditionsof anodization are shown below. It is to be noted, however, that thestarting material for producing porous Si by anodization is not limitedto monocrystalline Si and Si of other crystalline structure may be usedas the starting material.

Voltage applied: 2.6 (V)

Current density: 30 (mA·cm⁻²)

Anodizing solution: HF:H₂O:C₂H₅OH=1:1:1

Time: 2.4 hours

Thickness of porous Si: 300 (μm)

Porosity: 56 (%)

Test pieces of the porous Si thus prepared were immersed in 49%hydrofluoric acid solution (white circles) and 20% hydrofluoric acidsolution (black circles) both at the room temperature, and the solutionswere agitated. The reduction in the thickness of these test pieces ofthe porous Si were then measured. A large etching rate was observed:namely, 90 μm in 40 minutes in case of the 49% hydro hydro acid solutionand 72 μm in 40 minutes in case of the 20% hydrofluoric acid solution.After elapse of 80 minutes, the porous Si test pieces were etched by 205μm with a high degree of surface state in case of the 49% hydrofluoricacid solution, whereas, in case of the 20% hydrofluoric acid solution,the etching depth was 164 μm after elapse of 80 minutes.

The etching rate has dependencies on the concentration of the etchingsolution and the temperature. The etching solution concentration andtemperature are suitably determined i practical ranges. Althoughsolution concentration of 49% and 20% and the room temperature arementioned above, these are only illustrative and are not intended forlimiting purpose. Preferably, the concentration of the hydrofluoric acidranges between 5% and 95% and the temperature of the solution is set toa level which is ordinarily adopted in this field of technology.

The etched test pieces of porous Si were then rinsed with water and thesurfaces after the rinsing were examined by microanalysis usingsecondary ions but no impurity was detected.

A description will now be given with specific reference to FIG. 6A ofthe etching characteristics of porous Si and non-porous Si whenhydrofluoric acid is used as the etching solution, followed by adescription of an example of etching of a non-porous Si substrate whoseone side surface is completely porous Si, taken in conjunction withFIGS. 1A and 1B.

FIG. 6A is a graph showing the time dependency of etching depth ofporous Si and monocrystalline Si as observed when they are immersed inhydrofluoric acid solution. Porous Si was formed by anodizingmonocrystalline Si under the same conditions as those mentioned above.In this case also, the use of monocrystalline Si as the startingmaterial for anodization is only illustrative and Si of othercrystalline structures may be used as the starting material.

Test pieces of the porous Si thus prepared were immersed in 49%hydrofluoric acid solution (white circles) at the room temperature, andthe solutions were agitated. The reduction in the thickness of thesetest pieces of porous Si were then measured. A large etching rate wasobserved: namely, 90 μm in 40 minutes in case of the 49% hydrofluoricacid solution and, after elapse of 80 minutes, the porous Si test pieceswere etched by 205 μm with a high degree of surface state. The etchingrate has dependencies on the concentration of the etching solution andthe temperature. The etching solution concentration and temperature aresuitably determined in practical ranges. Although solution density of49% and the room temperature are mentioned above, these are onlyillustrative and are not intended for limiting purpose. Preferably, theconcentration of the hydrofluoric acid ranges between 5% and 95% and thetemperature of the solution is set to a level which is ordinarilyadopted in this field of technology.

A test piece of a non-porous Si of 500 μm thick was immersed in a 49%solution of hydrofluoric acid (black circles), followed by an agitationof the solution. The reduction in the thickness was then measured. Inthis case, the test piece of non-porous Si was etched only by 100Angstrom or less even after elapse of 120 minutes. The etching rateshowed almost no dependency on solution concentration and temperature.

Both the porous and non-porous Si test pieces after the etching wererinsed with water and the surface states of these test pieces wereexamined by microanalysis with secondary ions but no impurity wasdetected.

As shown in FIG. 1A, a monocrystalline Si substrate 22 was anodized onlyat its one side so as to have a porous Si structure only at its one sideas denoted by 21. Then, the substrate having the porousSi/monocrystalline Si structure was immersed in a hydrofluoric acid. Asa consequence, only the porous Si portion was removed by the etchingwhile the monocrystalline Si substrate 22 alone remained unetched. It isthus possible to selectively etch porous Si by using monocrystalline Sias the etch stopper.

A description will now be given of a case where both porous Si portionand monocrystalline Si portion are provided on one side of thesubstrate.

As shown in FIG. 2A, a portion of one side of a monocrystalline Sisubstrate 32 was anodized to become porous Si structure 31. Since thecurrent and voltage necessary for the anodization vary depending on thecarrier concentration, it is possible to selectively form porous Sistructure by locally varying the carrier concentration in themonocrystalline Si surface layer through implantation of proton orimpurities. The substrate having the porous Si/monocrystalline Sistructure was then immersed in hydrofluoric acid. As a result, only theporous Si portion was removed while the monocrystalline Si substrate 32remained unetched. It is thus possible to selectively etch porous Si.

A description will be made as to the case where a porous Si structureand a polycrystalline structure are formed in layers on one side of thesubstrate.

As shown in FIG. 3A, a polycrystalline Si layer 41 was formed bydeposition on a single-crystalline Si substrate 42, and the surfacelayer of this polycrystalline Si was changed into a porous Si layer 43by anodization, as shown in FIG. 3B. The substrate having the porousSi/polycrystalline Si/monocrystalline Si structure was immersed in asolution of hydrofluoric acid, whereby the porous Si structure alone wasremoved by etching while the monocrystalline Si substrate 42 and thepolycrystalline Si layer 41 remained unetched. It was thus possible toselectively etch the porous Si by using polycrystalline Si as theetching stopper.

A description will now be given of a case where porous Si portion isformed in the surface of a polycrystalline Si layer which is formed onone side of the substrate.

As shown in FIG. 4A, a polycrystalline Si layer 51 was formed bydeposition in a monocrystalline Si substrate 52 and a portion of thispolycrystalline Si layer was changed into porous Si layer 53 byanodization. Then, the substrate having the porous Si/polycrystallineSi/monocrystalline Si structure was immersed in a solution ofhydrofluoric acid, so that the porous Si alone was removed while themonocrystalline Si substrate 52 and the polycrystalline Si layer 51remained unetched. It was thus possible to selectively etch the porousSi

I-(2)

A description will now be given of the case where a mixture ofhydrofluoric acid and an alcohol is used as the electroless wet chemicaletching solution for porous Si, with reference to FIG. 7B.

FIG. 7B shows the time dependency of etching thickness of porous Si asobserved when the porous Si is immersed in a mixture liquid ofhydrofluoric acid and ethyl alcohol without agitation of the liquid. Theporous Si was formed by anodizing monocrystalline Si under theconditions shown below. The use of the monocrystalline Si as thestarting material for forming the porous Si structure throughanodization is only illustrative and Si of other crystalline structurescan be used as the starting material.

Voltage applied: 2.6 (V)

Current density: 30 (mA·cm⁻²)

Anodizing solution: HF:H₂O: C₂H₅OH=1:1:1

Time: 2.4 (hours)

Thickness of porous Si: 300 (μm)

Porosity: 56 (%)

Test pieces of the porous Si prepared as described above were immersed,without agitation, in a mixture solution of 49% hydrofluoric acid andethyl alcohol (10:1) (white circles) and in a mixture solution of 20%hydrofluoric acid and ethyl alcohol (10:1) (black circles). Thereductions in the thicknesses of the porous Si test pieces were thenmeasured. Large rates of etching of the porous Si were observed: namely,in the case of the mixture solution of 49% hydrofluoric acid and ethylalcohol (10:1), the porous Si was etched by 85 μm and, in case of themixture solution of 20% hydrofluoric acid and ethyl alcohol (10:1), theporous Si was etched by 68 μm, in about 40 minutes. After elapse of 80minutes, the porous Si was etched by a thickness as large as 195 μm inthe case of the mixture solution of 49% hydrofluoric acid and ethylalcohol (10:1) and 156 μm even in the case of the mixture solution of20% hydrofluoric acid and ethyl alcohol (10:1), with high degrees ofstates of the etched surfaces.

The etching rate has dependencies on the concentration of thehydrofluoric acid solution, as well as on the temperature. The additionof alcohol serves to remove bubbles of reaction product gases generatedas a result of the etching without delay from the surface being etched,without necessitating agitation, thus ensuring a high efficiency anduniformity of the etching.

The solution concentration and the temperature are determined such thata practical etching speed is obtained in preparation process and theeffect of addition of alcohol is appreciable. Although the mixturesolutions of 49% hydrofluoric acid and ethyl alcohol (10:1) and 20%hydrofluoric acid and ethyl alcohol (10:1), as well as the roomtemperature as the solution temperature, are mentioned, these solutiondensities and temperature are only illustrative and are not intended torestrict the scope of the invention. The HF concentration with respectto the etching solution preferably ranges between 1 and 95 %, morepreferably between 5 and 90% and most preferably between 5 and 80%. Theconcentration of alcohol with respect to the etching solution ispreferably 80% or less, more preferably 60% or less and most preferably40% or less, and is determined so as to provide an appreciable effect ofaddition of the alcohol. The temperature is selected to range preferably0 to 100° C., more preferably 5 to 800° C. and most preferably 5 to 600°C.

Although ethyl alcohol has been mentioned specifically, the inventiondoes not exclude the use of other alcohol such as isopropyl alcoholwhich does not cause any inconvenience in the production process andwhich can provide an appreciable effect of addition of such alcohol.

The porous Si after the etching was rinsed with water and the rinsedsurface was examined by microanalysis by using secondary ions but noimpurity was detected.

A description will now be given of the etching characteristic of porousSi and non-porous Si when they are etched by a mixture solution ofhydrofluoric acid and ethyl alcohol, with specific reference to FIG. 6B.

FIG. 6B shows time dependencies of etched thicknesses of porous Si andmonocrystalline Si as observed when the porous Si and themonocrystalline Si are immersed in a mixture solution of hydrofluoricacid and ethyl alcohol without agitation. The porous Si was formed byanodization of monocrystalline Si conducted under the same conditions asthose shown before. The use of monocrystalline Si as the startingmaterial for forming porous Si through anodization is only illustrativeand Si of other crystalline structures can be used as the startingmaterial.

A test piece of porous Si prepared as described above was immersed,without agitation, in a mixture solution of 49% hydrofluoric acid andethyl alcohol(10:1) (while circles), and reduction in the thickness ofthe porous Si was measured. The porous Si was rapidly etched: namely, bya thickness of 85 μm in 40 minutes and 195 μm in 80 minutes, with highdegrees of surface quality and uniformity.

The etching rate depends on the concentration and the temperature of thehydrofluoric acid solution.

The addition of alcohol serves to remove bubbles of reaction productgases generated as a result of the etching without delay from thesurface being etched, without necessitating agitation, thus ensuring ahigh efficiency and uniformity of the etching.

A test piece of a non-porous Si of 500 μm thick was immersed in amixture solution of 49 hydrofluoric acid and ethyl alcohol (10 1) (blackcircles), without agitation of the solution. The reduction in thethickness was then measured. In this case, the test piece of non-porousSi was etched only by 100 Angstrom or less even after elapse of 120minutes. The etching rate showed almost no dependency on the solutionconcentration and temperature.

Both the porous and non-porous Si test pieces after the etching wererinsed with water and the surface states of these test pieces wereexamined by microanalysis with secondary ions but no impurity wasdetected.

Naturally, various etching methods explained in connection with I bymaking reference to FIGS. 1A and 1B, FIGS. 2A and 2B, FIGS. 3A to 3C andFIGS. 4A to 4C can be realized also in the case where the mixturesolution of hydrofluoric acid and an alcohol is used as the etchant forporous Si.

I-(3)

A description will now be given of the case where a mixture ofhydrofluoric acid and aqueous hydrogen peroxide (hereinafter alsoreferred to as “hydrogen peroxide”) is used as the electroless wetchemical etching solution for porous Si, with reference to FIG. 7C.

FIG. 7C shows the time dependency of etched thickness of porous Si asobserved when the porous Si is immersed in a mixture liquid ofhydrofluoric acid and hydrogen peroxide under agitation of the liquid.The porous Si was formed by anodizing monocrystalline Si under theconditions shown below. The use of the monocrystalline Si as thestarting material for forming the porous Si structure throughanodization is only illustrative and Si of other crystalline structurescan be used as the starting material.

Voltage applied: 2.6 (V)

Current density: 30 (mA·cm⁻²)

Anodizing solution: HF:H₂O:C₂H₅OH=1:1:1

Time: 2.4 (hours)

Thickness of porous Si: 300 (μm)

Porosity: 56 (%)

Test pieces of the porous Si prepared as described above were immersed,without agitation, in a mixture solution of 49% hydrofluoric acid andhydrogen peroxide (1:5) (white circles) and in a mixture solution of 49%hydrofluoric acid and hydrogen peroxide (1:1) (black circles). Thereductions in the thicknesses of the porous Si test pieces were thenmeasured. Large rates of etching of the porous Si were observed: namely,in the case of the 1:5 solution, the porous Si was etched by 112 μm and,in case of the 1:1 solution, the porous Si was etched by 135 μm, inabout 40 minutes. After elapse of 80 minutes, the porous Si was etchedby a thickness as large as 256 μm in the case of the 1:5 solution and307 μm in the case of the 1:1 solution, with high degrees of states ofthe etched surfaces. The concentration of hydrogen peroxide was 30% inthis case but the hydrogen peroxide concentration may be determined in arange which provides an appreciable effect of addition of hydrogenperoxide and which does not cause any practical problem in theproduction process.

The etching rate has dependencies on the density of the hydrofluoricacid solution, as well as on the temperature of the same. The additionof alcohol serves to accelerate oxidation of silicon, thus enhancing thereaction speed as compared to the case where hydrogen peroxide is notused. It is also possible to control the reaction speed by suitablyselecting the content of hydrogen peroxide.

The solution concentration and the solution temperature are determinedsuch that a practical etching speed is obtained in preparation processand the effect of hydrofluoric acid and hydrogen peroxide isappreciable. Although the mixture solutions of 49% hydrofluoric acid andhydrogen peroxide (1:5) and 49% hydrofluoric acid and hydrogen peroxide(1:1), as well as the room temperature as the solution temperature, arementioned, these solution concentrations and temperature are onlyillustrative and are not intended to restrict the scope of theinvention.

The HF concentration with respect to the etching solution preferablyranges between 1 and 95%, more preferably between 5 and 90% and mostpreferably between 5 and 80%. The concentration of H₂O₂ with respect tothe etching solution is preferably selected to range between 1 and 95%,more preferably between 5 and 90% and most preferably between 10 and80%, and is determined so as to provide an appreciable effect ofaddition of the hydrogen peroxide. The temperature is selected to rangepreferably 0 to 100° C., more preferably 5 to 80° C. and most preferably5 to 60° C.

The porous Si after the etching was rinsed with water and the rinsedsurface was examined by microanalysis by using secondary ions but noimpurity was detected.

A description will now be given of the etching characteristics of porousSi and non-porous Si as observed when they are etched by a mixturesolution of hydrofluoric acid and aqueous hydrogen peroxide, withspecific reference to FIG. 6C. FIG. 6C shows the time dependencies ofetched depth of porous Si and monocrystalline Si as observed when theporous Si and the monocrystalline Si are immersed in a mixture solutionof hydrofluoric acid and aqueous hydrogen peroxide. The porous Si wasformed by anodization of monocrystalline Si conducted under the sameconditions as those shown before. The use of monocrystalline Si as thestarting material for forming porous Si through anodization is onlyillustrative and Si of other crystalline structures can be used as thestarting material.

A test piece of porous Si prepared as described above was immersed,followed by agitation, in a mixture solution of 49% hydrofluoric acidand aqueous hydrogen peroxide (white circles), and reduction in thethickness of the porous Si was measured. The porous Si was rapidlyetched: namely, by a thickness of 112 μm in 40 minutes and 256 μm in 80minutes, with high degrees of surface quality and uniformity. Althoughthe concentration of aqueous hydrogen peroxide was 30% in this case, theconcentration of hydrogen peroxide may be suitably determined within arange which does not impair the effect of addition of hydrogen peroxideand which does not cause any practical inconvenience in the production.

The etching rate depends on the concentration and the temperature of thehydrofluoric acid and aqueous hydrogen peroxide

The addition of hydrogen peroxide serves to

accelerate oxidation of silicon, thus enhancing the reaction speed ascompared to the case where hydrogen peroxide is not added. Furthermore,the reaction speed can be controlled by suitably selecting the contentof the hydrogen peroxide.

A test piece of a non-porous Si of 500 μm thick was immersed in amixture solution of 49% hydrofluoric acid and aqueous hydrogen peroxide(1:5) (Black circles), followed by agitation of the solution. Thereduction in the thickness of the porous Si was then measured. In thiscase, the test piece of nonporous Si was etched only by 100 Angstrom orless even after elapse of 120 minutes. The etching rate showed almost nodependency on the solution concentration and temperature.

Both the porous and non-porous Si test pieces after the etching wererinsed with water and the surface states of these test pieces wereexamined by microanalysis with secondary ions but no impurity wasdetected.

Naturally, various embodiments explained in connection with I by makingreference to FIGS. 1A and 1B, FIGS. 2A and 2B, FIGS. 3A to 3C and FIGS.4A to 4C can be realized also in the case where the mixture solution ofhydrofluoric acid and aqueous hydrogen peroxide is used as the etchingsolution.

I-(4)

A description will now be given of the case where a mixture ofhydrofluoric acid, an alcohol and aqueous hydrogen peroxide is used asthe electroless wet chemical etching solution for porous Si, withreference to FIG. 7D.

FIG. 7D shows the time dependency of etched thickness of porous Si asobserved when the porous Si is immersed in a mixture liquid ofhydrofluoric acid, ethyl alcohol and aqueous hydrogen peroxide withoutagitation of the liquid. The porous Si was formed by anodizingmonocrystalline Si under the conditions shown below. The use of themonocrystalline Si as the starting material for forming the porous Sistructure through anodization is only illustrative and Si of othercrystalline structures can be used as the starting material.

Voltage applied: 2.6 (V)

Current density: 30 (mA·cm⁻²

Anodizing solution: HF:H₂O:C₂H₅OH=1:1:1

Time: 2.4 (hours)

Thickness of porous Si: 300 (μm)

Porosity: 56 (%)

Test pieces of the porous Si prepared as described above were immersed,without agitation, in a mixture solution of 49% hydrofluoric acid, ethylalcohol and aqueous hydrogen peroxide (10:6:50) (white circles) and in amixture solution of 49% hydrofluoric acid, ethyl alcohol and hydrogenperoxide (10:2:10) (black circles). The reductions in the thicknesses ofthe porous Si test pieces were then measured. Large rates of etching ofthe porous Si were observed: namely, in the case of the 10:6:50solution, the porous Si was etched by 107 μm and, in case of the 10:2:10solution, the porous Si was etched by 128 μm, in about 40 minutes. Afterelapse of 80 minutes, the porous Si was etched by a thickness as largeas 244 μm in the case of the 10:6:50 solution and 292 μm in the case ofthe 10:2:10 solution, with high degrees of states of the etchedsurfaces. The concentration of aqueous hydrogen peroxide was 30% in thiscase but the hydrogen peroxide concentration may be determined in arange which provides an appreciable effect of addition of hydrogenperoxide and which does not cause any practical problem in theproduction process.

The etching rate has dependencies on the density of the hydrofluoricacid solution, as well as on the temperature of the same. The additionof alcohol serves to accelerate oxidation of silicon, thus enhancing thereaction speed as compared to the case where hydrogen peroxide is notused. It is also possible to control the reaction speed by suitablyselecting the content of hydrogen peroxide. On the other hand, theaddition of alcohol serves to remove, without delay, bubbles of reactiongases generated as a result of the etching from the etched surface,without requiring agitation of the solution, thus offering high degreesof efficiency and uniformity of etching of the porous Si.

The solution density and the solution temperature are determined suchthat a practical etching speed is obtained and such that the effect ofthe use of hydrofluoric acid, alcohol and hydrogen peroxide isappreciable. Although the mixture solutions of 49% hydrofluoric acid,ethyl alcohol and hydrogen peroxide (10:6:50) and 49% hydrofluoric acid,ethyl alcohol and hydrogen peroxide (10:2:10), as well as the roomtemperature as the solution temperature, are mentioned, these solutiondensities and temperature are only illustrative and are not intended torestrict the scope of the invention.

The HF concentration with respect to the etching solution preferablyranges between 1 and 95%, more preferably between 5 and 90% and mostpreferably between 5 and 80%. The concentration of H₂O₂ with respect tothe etching solution is preferably selected to range between 1 and 95%,more preferably between 5 and 90% and most preferably between 10 and80%, and is determined so as to provide an appreciable effect ofaddition of the hydrogen peroxide. The concentration of the alcohol withrespect to the etching solution is preferably determined to be 80% orless, more preferably 60% or less and most preferably 40% or less, andis selected so as to provide an appreciable effect of addition of thealcohol. The temperature is selected to range preferably 0 to 100° C.,more preferably 5 to 80° C. and most preferably 5 to 60° C.

Alcohol to be used in the present invention is not limited to ethylalcohol and includes those alcohols such as isopropyl alcohol which canpractically be used in preparation process and accomplish the effect ofthe addition of alcohol as mentioned above.

The porous Si after the etching was rinsed with water and the rinsedsurface was examined by microanalysis by using secondary ions but noimpurity was detected.

This type of etching solution is advantageous in that bubbles ofreaction product gases generated as a result of the etching can beremoved without delay from the surface being etched, without requiringagitation, so that the surface is etched with high degrees of smoothnessand uniformity even to the bottoms of minute recesses which may exist inthe etched surface.

A description will now be given of the etching characteristics of porousSi and non-porous Si as observed when they are etched by a mixturesolution of hydrofluoric acid, ethyl alcohol and aqueous hydrogenperoxide, with specific reference to FIG. 6D.

FIG. 6D shows the time dependencies of etched thickness of porous Si andmonocrystalline Si as observed when the porous Si and themonocrystalline Si are immersed in a mixture solution of hydrofluoricacid, ethyl alcohol and aqueous hydrogen peroxide, without agitation.The porous Si was formed by anodization of monocrystalline Si conductedunder the same conditions as those shown before. The use ofmonocrystalline Si as the starting material for forming porous Sithrought anodization is only illustrative and Si of other crystallinestructures can be used as the starting material.

A test piece of porous Si prepared as described above was immersed,without agitaion, in a mixture solution of 49% hydrofluoric acid, ethylalcohol and aqueous hydrogen peroxide (10:6:50) (while circles) at theroom temperature, and reduction in the thickness of the porous Si wasmeasured. The porous Si was rapidly etched: namely, by a thickness of107 μm in 40 minutes and 244 μm in 80 minutes, with high degrees ofsurface quality and uniformity. Although the concentration of theaqueous hydrogen peroxide was 30% in this case, the content of hydrogenperozide may be suitably determined within a range which does not impairthe effect of addition of hydrogen peroxide and which does not cause anypractical inconvenience in the production.

The etching rate depends on the concentration and the temperature of thehydrofluoric acid and aqueous hydrogen peroxide.

The addition of hydrogen peroxide serves to accelerate oxidation ofsilicon, thus enhancing the reaction speed as compared to the case wherehydrogen peroxide is not added. Furthermore, the reaction speed can becontrolled by suitably selecting the content of the hydrogen peroxide.In addition, alcohol serves to remove, without delay, bubbles of thereaction product gases generated as a result of the etching withoutrequiring agitation, thus ensuring high degrees of uniformity andetching of the porous Si.

A test piece of a non-porous Si of 500 μm thick was immersed in amixture solution of 49% hydrofluoric acid, ethyl alcohol and aqueoushydrogen peroxide (10:6:50) (black circles) at the room temperature,without agitation of the solution. The reduction in the thickness wasthen measured. In this case, the test piece of non-porous Si was etchedonly by 100 Angstrom or less even after elapse of 120 minutes. Theetching rate showed almost no dependency on the solution concentrationand temperature.

Both the porous and non-porous Si test pieces after the etching wererinsed with water and the surface states of these test pieces wereexamined by microanalysis with secondary ions but no impurity wasdetected.

Naturally, the various embodiments explained in connection with I bymaking reference to FIGS. 1A and 1B, FIGS. 2A and 2B, FIGS. 3A to 3C andFIGS. 4A to 4C can be realized also in the case where the mixturesolution of hydrofluoric acid, alcohol and aqueous hydrogen peroxide isused as the etchant for porous Si.

I-(5)

A description will now be given of the case where a bufferedhydrofluoric acid is used as the electroless wet chemical etchingsolution for porous Si, with reference to FIG. 7E. For instance, anaqueous solution containing 36.2 % of ammonium fluoride (NH₄F) and 4.5%of hydrogen fluoride (HF) is used as the buffered hydrofluoric acid.

FIG. 7E shows the time dependency of etching thickness of porous Si asobserved when the porous Si is immersed in the buffered hydrofluoricacid followed by agitation. The porous Si was formed by anodizingmonocrystalline Si under the conditions shown below. The use of themonocrystalline Si as the starting material for forming the porous Sistructure through anodization is only illustrative and Si of othercrystalline structures can be used as the starting material.

Voltage applied: 2.6 (V)

Current density: 30 (mA·cm⁻²

Anodizing solution: HF:H₂O:C₂H₅OH=1:1:1

Time: 2.4 (hours)

Thickness of porous Si: 300 (μm)

Porosity: 56 (%)

Test pieces of the porous Si prepared as described above were immersed,followed by agitation, in the buffered hydrofluoric acid (white circles)and in a 20% diluted buffered hydrofluoric acid (black circles). Thereductions in the thicknesses of the porous Si test pieces were thenmeasured. Large rates of etching of the porous Si were observed: namely,in the case of buffered hydrofluoric acid, the porous Si was etched by70 μm and, in case of the 20% diluted buffered hydrofluoric acid, theporous Si was etched by 56 μm, in about 40 minutes. After elapse of 120minutes, the porous Si was etched by a thickness as large as 118 μm inthe case of the buffered hydrofluoric acid and 94 μm in the case of the20% diluted buffered hydrofluoric acid, with high degrees of states ofthe etched surfaces.

The etching rate has dependencies on the density of the hydrofluoricacid solution, as well as on the temperature of the same. The density ofthe solution and the temperature of the same are determined to fallwithin the ranges which would not cause any practical inconvenience.Although the buffered hydrofluoric acid which is an aqueous solutioncontaining 36.2% of ammonium fluoride (NH₄F) and 4.5% of hydrogenfluoride (HF) and the 20% diluted buffered hydrofluoric acid, as well asthe room temperature as the solution temperature, are mentioned, thesesolution densities and temperature are only illustrative and are notintended to restrict the scope of the invention.

The HF concentration in the buffered hydrofluoric acid with respect tothe etching solution preferably ranges between 1 and 95%, morepreferably between 1 and 85% and most preferably between 1 and 70%. Theconcentration of NH₄ in the buffered hydrofluoric acid with respect tothe etching solution is preferably selected to range between 1 and 95%,more preferably between 5 and 90% and most preferably between 5 and 80%.The temperature is selected to range preferably 0 to 100° C., morepreferably 5 to 80° C. and most preferably 5 to 60° C.

The porous Si after the etching was rinsed with water and the rinsedsurface was examined by microanalysis by using secondary ions but noimpurity was detected.

A description will now be given of the etching characteristics of porousSi and non-porous Si as observed when they are etched by the bufferedhydrofluoric acid, with specific reference to FIG. 6E. FIG. 6E shows thetime dependencies of etching of porous Si and monocrystalline Si asobserved when the porous Si and the monocrystalline Si are immersed inthe buffered hydrofluoric acid. The porous Si was formed by anodizationof monocrystalline Si conducted under the same conditions as those shownbefore. The use of monocrystalline Si as the starting material forforming porous Si through anodization is only illustrative and Si ofother crystalline structures can be used as the starting material.

A test piece of porous Si prepared as described above was immersed,followed by agitation, in the buffered hydrofluoric acid (white circles)at the room temperature, and reduction in the thickness of the porous Siwas measured. The porous Si was rapidly etched: namely, by a thicknessof 70 μm in 40 minutes and 118 μm in 120 minutes, with high degrees ofsurface quality and uniformity.

The etching rate has dependencies on the density of the hydrofluoricacid solution, as well as on the temperature of the same. The density ofthe solution and the temperature of the same are determined to fallwithin the ranges which would not cause any practical inconvenience.Although the buffered hydrofluoric acid which is an aqueous solutioncontaining 36.2% of ammonium fluoride (NH₄F) and 4.5% of hydrogenfluoride (HF) as well as the room temperature as the solutiontemperature, are mentioned, these solution densities and temperature areonly illustrative and are not intended to restrict the scope of theinvention.

A test piece of a non-porous Si of 500 μm thickness was immersed in thebuffered hydrofluoric acid(black circles) at the room temperature,followed by agitation of the solution. The reduction in the thicknesswas then measured. In this case, the test piece of non-porous Si wasetched only by 100 Angstroms or less even after elapse of 120 minutes.The etching rate showed almost no dependency on the solution density andtemperature.

Both the porous and non-porous Si test pieces after the etching wererinsed with water and the surface states of these test pieces wereexamined by microanalysis with secondary ions but no impurity wasdetected.

Obviously, various etching methods explained in connection with (1) bymaking reference to FIGS. 1A and 1B, FIGS. 2A and 2B, FIGS. 3A to 3C andFIGS. 4A to 4C can be realized also in the case where the bufferedhydrofluoric acid is used as the etching solution.

I-(6)

A description will now be given of the case where a mixture liquid of abuffered hydrofluoric acid and an alcohol is used as the electroless wetchemical etching solution for porous Si, with reference to FIG. 7F. Forinstance, an aqueous solution containing 36.2% of ammonium fluoride(NH₄F) and 4.5% of hydrogen fluoride (HF) is used as the bufferedhydrofluoric acid.

FIG. 7F shows the time dependency of etching thickness of porous Si asobserved when the porous Si is immersed in the mixed solution of thebuffered hydrofluoric acid and ethyl alcohol, without agitation. Theporous Si was formed by anodizing monocrystalline Si under theconditions shown below. The use of the monocrystalline Si as thestarting material for forming the porous Si structure throughanodization is only illustrative and Si of other crystalline structurescan be used as the starting material.

Voltage applied: 2.6 (V)

Current density: 30 (mA·cm⁻²)

Anodizing solution: HF:H₂O:C₂H₅OH=1:1:1

Time: 2.4 (hours)

Thickness of porous Si: 300 (μm)

Porosity: 56 (%)

Test pieces of the porous Si prepared as described above were immersed,without agitation, in a mixture solution of the buffered hydrofluoricacid and ethyl alcohol(10:1) (white circles) and in a mixture solutionof 20% diluted buffered hydrofluoric acid and ethyl alcohol (10:1)(black circles). The reductions in the thicknesses of the porous Si testpieces were then measured. Large rates of etching of the porous Si wereobserved: namely, in the case of the mixture solution of the bufferedhydrofluoric acid and ethyl alcohol (10:1), the porous Si was etched by67 μm and, in case of the mixture solution of the 20% diluted bufferedhydrofluoric acid and ethyl alcohol (10:1), the porous Si was etched by54 μm, in about 40 minutes. After elapse of 120 minutes, the porous Siwas etched by a thickness as large as 112 μm in the case of the mixturesolution of the buffered hydrofluoric acid and ethyl alcohol (10:1) and90 μm in the case of the mixture solution of 20% diluted bufferedhydrofluoric acid and ethyl alcohol (10:1), with high degrees of statesof the etched surfaces. The etching rate has dependencies on the densityof the hydrofluoric acid solution, as well as on the temperature of thesame. The addition of alcohol serves to remove, without delay, bubblesof reaction product gases generated as a result of the etching from thesurface being etched, without requiring agitation, thus enabling etchingof the porous Si with high degrees of uniformity and efficiency.

The density of the solution and the temperature of the same aredetermined to fall within the ranges which would not cause any practicalinconvenience. Although the mixture solution of the bufferedhydrofluoric acid and ethyl alcohol (10:1) and the mixture solution ofthe 20% diluted buffered hydrofluoric acid and ethyl alcohol (10:1), aswell as the room temperature as the solution temperature, are mentioned,these solution densities and temperature are only illustrative and arenot intended to restrict the scope of the invention.

The HF concentration in the buffered hydrofluoric acid with respect tothe etching solution preferably ranges between 1 and 95%, morepreferably between 1 and 85% and most preferably between 1 and 70%. Theconcentration of NH₄ in the buffered hydrofluoric acid with respect tothe etching solution is preferably selected to range between 1 and 95%,more preferably between 5 and 90% and most preferably between 5 and 80%.The concentration of the alcohol with respect to the etching solution ispreferably 80% or less, more preferably 60% or less and most preferably40% or less, and is determined to make the effect of addition of thealcohol appreciable. The temperature is selected to range preferably 0to 100° C., more preferably 5 to 80° C. and most preferably 5 to 600° C.

Although ethyl alcohol has been specifically mentioned, other alcoholssuch as isopropyl alcohol, which does not cause any inconvenience in thecommercial production and which can provide an appreciable effect ofaddition of such alcohol, may be used as the alcohol used in this typeof etching solution.

The porous Si after the etching was rinsed with water and the rinsedsurface was examined by microanalysis by using secondary ions but noimpurity was detected.

In this etching solution, bubbles of reaction product gases generated asa result of the etching can be removed without delay and withoutrequiring agitation of the solution, by virtue of the addition of thealcohol, so that the bottoms of minute recesses can be formed with highdegrees of smoothness and uniformity.

A description will now be given of the etching characteristics of porousSi and non-porous Si as observed when they are etched by the mixturesolution of the buffered hydrofluoric acid and the ethyl alcohol, withspecific reference to FIG. 6F. FIG. 6F shows the time dependencies ofetching thickness of porous Si and monocrystalline Si as observed whenthe porous Si and the monocrystalline Si are immersed in the mixturesolution of the buffered hydrofluoric acid and ethyl alcohol. The porousSi was formed by anodization of monocrystalline Si conducted under thesame conditions as those shown before. The use of monocrystalline Si asthe starting material for forming porous Si through anodization is onlyillustrative and Si of other crystalline structures can be used as thestarting material.

A test piece of porous Si prepared as described above was immersed,without agitation, in the mixture solution of the buffered hydrofluoricacid and ethyl alcohol (10:1) (white circles) at the room temperature,and reduction in the thickness of the porous Si was measured. The porousSi was rapidly etched: namely, by a thickness of 67 μm in 40 minutes and112 μm in 120 minutes, with high degrees of surface quality anduniformity.

A test piece of a non-porous Si of 500 μm thickness was immersed in themixture solution of the buffered hydrofluoric acid and ethyl alcohol(10:1) (black circles) at the room temperature, without agitation of thesolution. The reduction in the thickness of the non-porous Si was thenmeasured. In this case, the test piece of non-porous Si was etched onlyby 100 Angstroms or less even after elapse of 120 minutes. The etchingrate showed almost no dependency on the solution density andtemperature.

Both the porous and non-porous Si test pieces after the etching wererinsed with water and the surface states of these test pieces wereexamined by microanalysis with secondary ions but no impurity wasdetected.

Obviously, various etching methods explained in connection with (1) bymaking reference to FIGS. 1A and 1B, FIGS. 2A and 2B, FIGS. 3A to 3C andFIGS. 4A to 4C can be realized also in the case where the mixturesolution of the buffered hydrofluoric acid and alcohol is used as theetching solution.

I-(7)

A description will now be given of the case where a mixture solution ofa buffered hydrofluoric acid and hydrogen peroxide is used as theelectroless wet chemical etching solution for porous Si, with referenceto FIG. 7G. For instance, an aqueous solution containing 36.2% ofammonium fluoride (NH₄F) and 4.5% of hydrogen fluoride (HF) is used asthe buffered hydrofluoric acid.

FIG. 7G shows the time dependency of etching thickness of porous Si asobserved when the porous Si is immersed in the mixed solution of thebuffered hydrofluoric acid and hydrogen peroxide followed by agitation.The porous Si was formed by anodizing monocrystalline Si under theconditions shown below. The use of the monocrystalline Si as thestarting material for forming the porous Si structure throughanodization is only illustrative and Si of other crystalline structurescan be used as the starting material.

Voltage applied: 2.6 (V)

Current density: 30 (mA·cm⁻²)

Anodizing solution: HF:H₂O:C₂H₅OH=1:1:1

Time: 2.4 (hours)

Thickness of porous Si: 300 (μm)

Porosity: 56 (%)

Test pieces of the porous Si prepared as described above were immersed,followed by agitation, in a mixture solution of the bufferedhydrofluoric acid and hydrogen peroxide (1:5) (white circles) and in amixture solution of the buffered hydrofluoric acid and hydrogen peroxide(5:1) (black circles). The reductions in the thicknesses of the porousSi test pieces were then measured. Large rates of etching of the porousSi were observed: namely, in the case of the 1:5 mixture solution, theporous Si was etched by 88 μm and, in case of the 5:1 mixture solution,the porous Si was etched by 105 μm, in about 40 minutes. After elapse of120 minutes, the porous Si was etched by a thickness as large as 147 μmin the case of the 1:5 mixture and 177 μm in the case of the Si mixturesolution, with high degrees of states of the etched surfaces. In thiscase, the concentration of hydrogen peroxide was 30%. This, however, isonly illustrative and the concentration of hydrogen peroxide is suitablyselected within a range which does not impair the effect of addition ofhydrogen peroxide. The etching rate has dependencies on the solutiondensities of the buffered hydrofluoric acid and hydrogen peroxide, aswell as on the temperature of the same. The addition of hydrogenperoxide accelerates the oxidation of silicon, thus attaining a higherreaction speed as compared to the case where hydrogen peroxide is notadded. In addition, the reaction speed can be controlled by suitablydetermining the content of hydrogen peroxide.

The density of the solution and the temperature of the same aredetermined to fall within the ranges which would not cause any practicalinconvenience in commercial production. Although the mixture solution ofthe buffered hydrofluoric acid and hydrogen peroxide (1:5) and themixture solution of the buffered hydrofluoric acid and hydrogen peroxide(5:1), as well as the room temperature as the solution temperature, arementioned, these solution densities and temperature are onlyillustrative and are not intended to restrict the scope of theinvention.

The HF concentration in the buffered hydrofluoric acid with respect tothe etching solution preferably ranges between 1 and 95%, morepreferably between 1 and 85% and most preferably between 1 and 70%. Theconcentration of NH₄ in the buffered hydrofluoric acid with respect tothe etching solution is preferably selected to range between 1 and 95%,more preferably between 5 and 90% and most preferably between 5 and 80%.The concentration of H₂O₂ with respect to the etching solution ispreferably 1 to 95 % more preferably 5 to 90% and most preferably 10 to80 %, and is determined to make the effect of addition of the hydrogenperoxide. The temperature is selected to range preferably 0 to 100° C.,more preferably 5 to 80° C. and most preferably 5 to 600C.

The porous Si after the etching was rinsed with water and the rinsedsurface was examined by microanalysis by using secondary ions but noimpurity was detected.

A description will now be given of the etching characteristics of porousSi and non-porous Si as observed when they are etched by the mixturesolution of the buffered hydrofluoric acid and hydrogen peroxide, withspecific reference to FIG. 6G. FIG. 6G shows the time dependencies ofetching thickness of porous Si and monocrystalline Si as observed whenthe porous Si and the monocrystalline Si are immersed in the mixturesolution of the buffered hydrofluoric acid hydrogen peroxide. The porousSi was formed by anodization of monocrystalline Si conducted under thesame conditions as those shown before. The use of monocrystalline Si asthe starting material for forming porous Si through anodization is onlyillustrative and Si of other crystalline structures can be used as thestarting material.

A test piece of porous Si prepared as described above was immersed,followed by agitation, in the mixture solution of the bufferedhydrofluoric acid and hydrogen peroxide (1:5) (white circles) at theroom temperature, and reduction in the thickness of the porous Si wasmeasured. The porous Si was rapidly etched: namely, by a thickness of 88μm in 40 minutes and 147 μm in 120 minutes, with high degrees of surfacequality and uniformity. Although in this case the concentration ofhydrogen peroxide was 30%, this is only illustrative and the content ofhydrogen peroxide is suitably selected within a range which does notcause any practical inconvenience and which does not impair the effectproduced by the addition of hydrogen peroxide.

Both the porous and non-porous Si test pieces after the etching wererinsed with water and the surface states of these test pieces wereexamined by microanalysis with secondary ions but no impurity wasdetected.

Obviously, various etching methods explained in connection with (1) bymaking reference to FIGS. 1A and 1B, FIGS. 2A and 2B, FIGS. 3A to 3C andFIGS. 4A to 4C can be realized also in the case where the mixturesolution of the buffered hydrofluoric acid and alcohol is used as theetching solution.

I-(8)

The following will now be given of the case where a mixture solution ofa buffered hydrofluoric acid, an alcohol and hydrogen peroxide is usedas the electroless wet chemical etching solution for porous Si, withreference to FIG. 7H. For instance, an aqueous solution containing 36.2%of ammonium fluoride (NH₄F) and 4.5% of hydrogen fluoride (HF) is usedas the buffered hydrofluoric acid.

FIG. 7H shows the time dependency of etching thickness of porous Si asobserved when the porous Si is immersed in the mixed solution of thebuffered hydrofluoric acid, ethyl alcohol and hydrogen peroxide withoutagitation. The porous Si was formed by anodizing monocrystalline Siunder the conditions shown below. The use of the monocrystalline Si asthe starting material for forming the porous Si structure throughanodization is only illustrative and Si of other crystalline structurescan be used as the starting material.

Voltage applied: 2.6 (V)

Current density: 30 (mA·cm⁻²)

Anodizing solution: HF:H₂O:C₂H₅OH=1:1:1

Time: 2.4 (hours)

Thickness of porous Si: 300 (μm)

Porosity: 56 (%)

Test pieces of the porous Si prepared as described above were immersed,without agitation, in a mixture solution of the buffered hydrofluoricacid, ethyl alcohol and hydrogen peroxide (10:6:50) (white circles) andin a mixture solution of the buffered hydrofluoric acid, ethyl alcoholand hydrogen peroxide (50:6:10) (black circles). The reductions in thethicknesses of the porous Si test pieces were then measured. Large ratesof etching of the porous Si were observed: namely, in the case of the10:6:50 mixture solution, the porous Si was etched by 83 μm and, in caseof the 50:6:10 mixture solution, the porous Si was etched by 100 μm, inabout 40 minutes. After elapse of 120 minutes, the porous Si was etchedby a thickness as large as 140 μm in the case of the. 10:6:50 mixtureand 168 μm in the case of the 50:6:10 mixture solution, with highdegrees of states of the etched surfaces. In this case, theconcentration of hydrogen peroxide was 30%. This, however, is onlyillustrative and the concentration of hydrogen peroxide is suitablyselected within a range which does not impair the effect of addition ofhydrogen peroxide. The etching rate has dependencies on theconcentrations of the buffered hydrofluoric acid and hydrogen peroxide,as well as on the temperature of the same. The addition of hydrogenperoxide accelerates the oxidation of silicon, thus attaining a higherreaction speed as compared to the case where hydrogen peroxide is notadded. In addition, the reaction speed can be controlled by suitablydetermining the ratio of hydrogen peroxide. The addition of alcoholenables bubbles of reaction products gases generated as a result of theetching to be removed from the surface being etched, without delay andwithout agitation, thus making it possible to etch the porous Siuniformly and with high efficiency.

The concentrations of the solution and the temperature of the solutionare determined to fall within the ranges which provide the above effectsof the use of the buffered hydrofluoric acid, hydrogen peroxide and thealcohol and which would not cause any practical inconvenience incommercial production. Although the mixture solution of the bufferedhydrofluoric acid, ethyl alcohol and hydrogen peroxide (10:6:50) and themixture solution of the buffered hydrofluoric acid, ethyl alcohol andhydrogen peroxide (50:6:10), as well as the room temperature as thesolution temperature, are mentioned, these solution ratio andtemperature are only illustrative and are not intended to restrict thescope of the invention.

The HF concentration in the buffered hydrofluoric acid with respect tothe etching solution preferably ranges between 1 and 95%, morepreferably between 1 and 85% and most preferably between 1 and 70%. Theconcentration of NH₄F in the buffered hydrofluoric acid with respect tothe etching solution is preferably selected to range between 1 and 95%,more preferably between 5 and 90% and most preferably between 5 and 80%.The concentration of H₂O₂ with respect to the etching solution ispreferably 1 to 95 %, more preferably 5 to 90% and most preferably 10 to80 %, and is determined to make the effect of addition of the alcoholappreciable. The concentration of the alcohol with respect to theetching solution is preferably 80% or less, more preferably 60% or lessand most preferably 40 t or less, and is determined to make the effectof addition of the alcohol appreciable. The temperature is selected torange preferably 0 to 100° C., more preferably 5 to 80° C. and mostpreferably 5 to 600C.

Although ethyl alcohol has been specifically mentioned, other alcoholssuch as isopropyl alcohol, which does not cause any inconvenience in thecommercial production and which can provide an appreciable effect ofaddition of such alcohol, may be used as the alcohol used in this typeof etching solution.

The porous Si after the etching was rinsed with water and the rinsedsurface was examined by microanalysis by using secondary ions but noimpurity was detected.

This etching solution enables bubbles of reaction product gasesgenerated by the etching to be removed from the surface being etched,without delay and without requiring agitation, so that the etching canbe performed with high degrees of smoothness and uniformity at thebottoms of minute recesses of the surface to be etched.

Clearly, embodiments explained in the above (1) by making reference toFIGS. 1A and 1B, FIGS. 2A and 2B, FIGS. 3A to 3C and FIGS. 4A to 4C canbe realized also in the case where the mixture solution of the bufferedhydrofluoric acid, alcohol and hydrogen peroxide is used as the etchingsolution.

II. The following will now be given of a process of the invention forproducing a semiconductor member.

As explained before, the first embodiment of the process for producingthe semiconductor member in accordance with the present invention hasthe following features.

Namely, the first embodiment of the process of the invention forproducing a semiconductor member comprises the steps of: forming amember having a non-porous silicon monocrystalline layer and a poroussilicon layer; bonding to the monocrystalline layer a member having aninsulating material surface; and removing by etching the porous siliconlayer by immersing it in hydrofluoric acid.

As explained before, the second embodiment of the process of theinvention for producing a semiconductor member uses the same steps asthose in the method of the first embodiment, except that, in place ofthe hydrofluoric acid used in the first embodiment, one of the second toeighth embodiments of the etching solutions mentioned before.

The third to sixth embodiments of the process of the invention forproducing a semiconductor member, which also were explained before, aremore practical embodiments of the first and the second embodiments ofthe process of the invention. The process of the present invention forproducing a semiconductor member will be described hereinafter with.reference to the third to sixth embodiments.

II-(1) The third embodiment of the process of the invention forproducing a semiconductor member will be described with reference to thedrawings.

Embodiment 1

The following will be first given of a method in which the whole memberis changed into porous structure and then a monocrystalline layer isformed on the porous structure by epitaxial growth method. FIGS. 8A to8C are schematic sectional views of the semiconductor memberillustrating each of steps of the process.

Referring to FIG. 8A, as the first step, an Si monocrystallinesemiconductor member 11 is prepared and is wholly changed into porousstructure and, then, an epitaxial growth method is applied to onesurface of the porous member, thereby forming a thin film ofmonocrystalline Si layer 12. The porous structure of Si member is formedby, for example, an anodization employing an HF solution. The initialmonocrystalline Si having the density of 2.33 g/cm³ can be changed intoa porous Si member the density of which can be varied within the rangebetween 1.1 and 0.6 g/cm³ by varying the HF concentration of the etchingsolution between 50% and 20%.

Referring now to FIG. 8B, a light-transmissive substrate 13, which istypically a glass sheet, is prepared and bonded on the surface of themonocrystalline Si layer 12 on the porous Si member. Subsequently, anSi₃N₄ layer 14 is formed by deposition as an anti-etching film to coverthe entire member composed of the layer 12 and the substrate 13 and theSi₃N₄ layer on the porous Si member 11 is removed. Although Si₃N₄ layeris suitably used as the anti-etching layer, it is possible to use othermaterials such as Apiezon wax as the material of the anti-etching layer.The porous Si member 11 is then immersed in the etching solution of thepresent invention and the solution is agitated so that only the porousSi is etched by electroless chemical etching, whereby a thinnednon-porous monocrystalline silicon layer 12 is left on thelight-transmissive substrate 13.

FIG. 8C shows the semiconductor member obtained by the present process.It will be said that, as a result of the removal of the anti-etchingSi₃N₄ layer 14 in the step shown in FIG. 8B, a monocrystalline Si layer12 having a crystallinity equivalent to that of a silicon wafer isformed on the light-transmissive substrate 13 with high degrees ofsmoothness and uniformity and with a small thickness, over a wide areacovering the whole surface of the wafer.

The semiconductor member thus obtained is advantageous from the viewpoint of production of an insulation-isolated electronic device.

Embodiment 2

The following will now be given of a process in which an N-type layer isformed before changing the initial member into porous structure and,subsequently to the formation of the P-type layer, a selectiveanodization is effected to change only the P-type substrate into porousstructure.

Referring to FIG. 9A, as the first step, a layer 32 of a low impurityconcentration is formed on the surface of a P-type Si monocrystallinesubstrate 31, by an epitaxial growth. Alternatively, an N-typemonocrystalline layer 32 may be formed on the surface of the P-type Simonocrystalline substrate 31 by ion-implantation of proton.

Then, as shown in FIG. 9B, the P-type Si monocrystalline substrate 31 ischanged into a porous Si substrate 33 by effecting, on the reverse sideof the P-type Si monocrystalline substrate 31, an anodization using, forexample, an HF solution. The initial monocrystalline Si having thedensity of 2.33 g/cm³ can be changed into a porous member the density ofwhich can be varied within the range between 1.1 and 0.6 g/cm³ byvarying the HF concentration of the etching solution between 50% and20%.

Referring now to FIG. 9C, a light-transmissive substrate 34, which istypically a glass sheet, is prepared and bonded on the surface of themonocrystalline Si layer 32 on the porous Si member. Subsequently, anSi₃N₄ layer 35 is formed by deposition as an anti-etching film to coverthe entire member composed of the layer 32 and the substrate 34 and theSi₃N₄ layer on the porous Si member 33 is removed. Although Si₃N₄ layeris suitably used as the anti-etching layer, it is possible to use othermaterials such as Apiezon wax as the material of the anti-etching layer.The porous Si substrate 33 is then immersed in the etching solution ofthe present invention and the solution is agitated so that only theporous Si is etched by electroless chemical etching, whereby a thinnednon-porous monocrystalline silicon layer 32 is left on thelight-transmissive substrate 34.

FIG. 9D shows the semiconductor member obtained by the present process.That is, as a result of the removal of the anti-etching Si₃N₄ layer inthe step shown in FIG. 9D, a monocrystalline Si layer 32 having acrystallinity equivalent to that of a silicon wafer is formed on thelight-transmissive substrate 34 with high degrees of smoothness anduniformity and with a small thickness, over a wide area covering thewhole surface of the wafer.

The semiconductor member thus obtained is advantageous from the viewpoint of production of an insulation-isolated electronic device.

According to the result of an observation by a transmission electronmicroscope, micro-pores of an average diameter of about 600 Angstrom areformed in the porous Si layer, so that the density of the layer has beenreduced half or below that of the monocrystalline Si.

Nevertheless, the monocrystallinity is still maintained, so that it ispossible to form a monocrystalline Si layer on the porous layer byepitaxial growth. When the temperature exceeds 1000° C., rearrangementof internal pores occurs, which impedes the acceleration of the etching.For this reason, the epitaxial growth of the Si layer is preferablyeffected by a low-temperature growth method such as, for example, amolecule-ray epitaxial growth method, a CVD method such as plasma CVDmethod, low-pressure CVD method or photo-CVD method, a bias sputtermethod or a liquid-phase growth method.

II-(2)

The fourth embodiment of the process of the invention of producing asemiconductor member will be described with reference to the drawings.

Embodiment 1

The following will be first given of a form in which the whole P- orhigh-density N-type substrate is changed into porous structure and thena monocrystalline layer is formed on the porous structure by epitaxialgrowth method. FIGS. 10A to 10C are schematic sectional views of thesemiconductor member illustrating each of steps of the process.

Referring to FIG. 10A, as the first step, an Si monocrystallinesemiconductor member 11 of P-type (or high-density N-type) is preparedand is wholly changed into porous structure and, then, an epitaxialgrowth is effected by a suitable method on the surface of the porousmember, thereby forming a thin film of monocrystalline Si layer 12. Theporous structure is formed by, for example, an anodization employing anHF solution. The initial monocrystalline Si having the density of 2.33g/cm³ can be changed into a porous member the density of which can bevaried within the range between 1.1 and 0.6 g/cm³ by varying the HFconcentration of the etching solution between 50% and 20%.

Referring now to FIG. 10B, another Si substrate 13 is prepared and aninsulating layer (silicon oxide layer) 14 is formed on the surface ofthis Si substrate 13. Subsequently, the surface of the insulating layer14 of the Si substrate 13 is bonded to the surface of themonocrystalline layer 12 on the porous Si substrate. The whole structure11-14 composed of the substrates and layers to 14 is then immersed inthe etching solution of the present invention and the solution isagitated so that only the porous Si is etched by electroless wetchemical etching, whereby a thinned non-porous monocrystalline siliconlayer 12 is left on the insulating layer 14.

FIG. 10C shows the semiconductor member obtained by the present process.That is, the monocrystalline Si layer 12 having a crystallinityequivalent to that of a silicon wafer is formed on the insulating layer14 on the Si substrate 13 with high degrees of smoothness and uniformityand with a small thickness, over a wide area covering the whole surfaceof the wafer.

The semiconductor member thus obtained is advantageous from the viewpoint of production of an insulation-isolated electronic device.

Embodiment 2

The following will now be given of a process in which an N-type layer isformed before changing the initial member into porous structure and,subsequently to the formation of the N-type layer, a selectiveanodization is effected to change only the P-type substrate or thehigh-density N-type substrate into porous structure. FIGS. 11A to 11Dshow, in schematic sectional views, the semiconductor member indifferent steps of the production process.

Referring to FIG. 11A, as the first step, a layer 22 of a low impurityconcentration is formed on the surface of a P-type (or high-densityN-type) Si monocrystalline substrate 21, by an epitaxial growthperformed by a suitable method. Alternatively, an N-type monocrystallinelayer 22 may be formed on the surface of the P-type Si monocrystallinesubstrate 21 by ion-implantation of proton.

Then, as shown in FIG. 11B, the P-type Si monocrystalline substrate 21is changed into a porous Si substrate 23 by effecting, on the reverseside of the P-type monocrystalline substrate 21, an anodization using,for example, an HF solution. The initial monocrystalline Si having thedensity of 2.33 g/cm³ can be changed into a porous member the density ofwhich can be varied within the range between 1.1 and 0.6 g/cm³ byvarying the HF concentration of the etching solution between 50% and20%.

Referring now to FIG. 11C, another Si substrate 24 is prepared and aninsulating layer 25 (silicon oxide layer) is formed on the surface ofthe Si substrate 24. Then, the insulating layer 25 on the Si substrate24 is bonded to the surface of the monocrystalline Si layer 22 on theporous substrate. Subsequently, the whole structure composed of thesubstrates and layers 22 to 25 is immersed in the etching solution ofthe present invention and the solution is agitated so that only theporous Si is etched by electroless chemical etching, whereby a thinnednon-porous monocrystalline silicon layer 22 is left on the insulatinglayer 25.

FIG. 11D shows the semiconductor member obtained by the present process.That is, a monocrystalline Si layer 22 having a crystallinity equivalentto that of a silicon wafer is formed on the insulating layer 25 withhigh degrees of smoothness and uniformity and with a small thickness,over a wide area covering the whole surface of the wafer.

The semiconductor member thus obtained is advantageous from the viewpoint of production of an insulation-isolated electronic device.

II-(3)

The fifth embodiment of the process of the invention will be describedwith reference to the drawings. A description will be first given of aform in which the whole Si substrate is changed into porous structureand then a monocrystalline layer is formed on the porous structure byepitaxial growth method.

Referring to FIG. 12A, as the first step, an Si monocrystallinesubstrate 11 is prepared and is wholly changed into porous structureand, then, an epitaxial growth is effected by a suitable method on thesurface of the porous substrate, thereby forming a thin filmmonocrystalline layer 12. The porous structure is formed by, forexample, an anodization employing an HF solution. The initialmonocrystalline Si having the density of 2.33 g/cm³ can be changed intoa porous Si layer the density of which can be varied within the rangebetween 1.1 and 0.6 g/cm³ by varying the HF concentration of the etchingsolution between 50% and 20%. The porous layer is tended to form in a Ptype Si substrate. A transmission electromicroscopic observation showedthat the porous Si layer thus formed has micropores of a mean diameterof about 600 Angstroms.

Referring now to FIG. 12B, a light-transmissive substrate 13, which istypically a glass sheet, is prepared. Then, the surface of themonocrystalline Si layer on the porous Si substrate is oxidized to forman oxide layer 14. The above-mentioned light-transmissive substrate 13is then bonded on the surface of the oxide layer 14. This oxide layerplays an important role formation of device. Namely, with such an oxidelayer, the interface level generated at the interface under the Siactive layer can be made lower as compared with the glass interface, sothat the characteristics of the electronic device can be remarkablyimproved.

Referring further to FIG. 12B, and Si₃N₄ layer 15 is deposited as ananti-etching film (protective material) to cover the entire membercomposed of the two substrates bonded together, and the Si₃N₄ layer onthe surface of porous Si substrate is removed. Although Si₃N₄ layer issuitably used as the anti-etching layer, it is possible to use othermaterials such as Apiezon wax as the material of the anti-etching layer.The porous Si substrate 11 is then immersed in the etching solution ofthe present invention with agitating so that only the porous Si isetched by electroless chemical etching, whereby a thinnedmonocrystalline silicon layer is left on the light-transmissivesubstrate 13.

FIG. 12C shows the semiconductor member obtained by the describedprocess. It will be seen that, as a result of the removal of theanti-etching Si₃N₄ layer 15 in the step shown in FIG. 12B, amonocrystalline Si layer 12 having a crystallinity equivalent to that ofa silicon wafer is formed on the light-transmissive substrate 13 withhigh degree of smoothness and uniformity and with a small thickness,over a wide area covering the whole surface of the wafer.

The semiconductor member thus obtained is advantageous from the viewpoint of production of an insulation-isolated electronic device.

II-(4)

The sixth embodiment of the process of the invention for producing asemiconductor member will now be described.

Embodiment 1

A description will be first given of a form in which the whole Sisubstrate is changed into porous structure and then a monocrystallinelayer is formed by epitaxial growth method.

FIGS. 13A to 13C illustrate successive steps of the first embodiment inaccordance with the invention.

Referring to FIG. 13A, as the first step, an Si monocrystallinesubstrate is prepared and is wholly changed into porous structure (11).Then, an epitaxial growth is effected by a suitable method on thesurface of the porous substrate, thereby forming a thin film ofmonocrystalline layer 12. The porous structure is formed by, forexample, an anodization employing an HF solution. The initialmonocrystalline Si having the density of 2.33 g/cm³ can be changed intoa porous Si layer the density of which can be varied within the rangebetween 1.1 and 0.6 g/cm³ by varying the HF concentration of the etchingsolution between 50% and 20%. A transmission electromicroscopicobservation showed that the porous Si layer thus formed has microporesof a mean diameter of about 600 Angstroms.

Referring now to FIG. 13B, another Si substrate 13 is prepared and aninsulating material 14 is formed on the surface. Then the Si substratehaving the insulating material 14 is bonded to the surface of an oxidelayer 15 which is formed on the monocrystalline Si layer carried by theporous Si substrate. The insulating material 14 may be a depositedsilicon oxide, nitride, nitrided oxide, or tantalum, not to mention theinsulating layer of Si. The bonding step may be conducted by adheringclosely the rinsed surfaces, and heating both substrate in an oxygenatmosphere or a nitrogen atmosphere. The oxide layer 15 is formed forthe purpose of reducing the interface level of the monocrystalline layer12 which is the final active layer.

Then, as shown in FIG. 13C, the porous Si substrate 11 is immersed inthe etching solution of the present invention and the solution isagitated, so that only the porous Si is etched by electroless wetchemical etching so as to leave a thinned monocrystalline Si layer onthe insulating material. FIG. 13C shows the semiconductor substrateobtained according to the present invention. As a result, amonocrystalline Si layer 12 having crystallinity equivalent to that of asilicon wafer is formed on the insulated substrate 13 through theintermediary of the insulating material 14 and the oxide layer 15, withhigh degrees of smoothness and uniformity and with a small thicknessover a wide area covering the whole surface of the wafer.

The semiconductor member thus obtained is advantageous from the viewpoint of production of an insulation-isolated electronic device.

Embodiment 2

A second embodiment will be described with reference to the drawings.

FIGS. 14A to 14D show, in schematic sectional views, the secondembodiment according to the present invention.

Referring to FIG. 14A, as the first step, a layer 32 of a low impurityconcentration is formed on the surface of a P-type Si monocrystallinesubstrate 31, by an epitaxial growth performed by a suitable method.Alternatively, an N-type monocrystalline layer 32 may be formed on thesurface of the P-type Si monocrystalline substrate 21 by implantation ofproton.

Then, as shown in FIG. 14B, the P-type Si monocrystalline substrate 31is changed into a porous Si substrate 33 by effecting, on the reverseside of the P-type Si monocrystalline substrate 31 by anodization using,for example, an HF solution. The initial monocrystalline Si having thedensity of 2.33 g/cm³ can be changed into a porous member the density ofwhich can be varied within the range between 1.1 and 0.6 g/cm³ byvarying the HF concentration of the etching solution between 50% and 20%. As explained before, this porous layer is formed in the P-typesubstrate.

Referring now to FIG. 14C, another Si substrate 34 is prepared and aninsulating layer 35 is formed on the surface of the Si substrate 34.Then, the Si substrate 34 having the insulating layer 35 is bonded tothe surface of the oxide layer 36 formed on the monocrystalline Si layeron the porous Si substrate. Then, the porous Si substrate is immersed inthe etching solution of the present invention and the solution isagitated so that only the porous Si is etched by electroless chemicaletching, whereby a thinned non-porous monocrystalline silicon layer isleft on the insulating layer.

FIG. 14D shows the semiconductor substrate obtained by the describedprocess. It will be seen that a monocrystalline Si layer 32 having acrystallinity equivalent to that of a silicon wafer is formed on theinsulated substrate 34 through the intermediary of the oxide layer 36and the insulation layer 35, with high degrees of smoothness anduniformity and with a small thickness, over a wide area covering thewhole surface of the wafer.

The semiconductor member thus obtained is advantageous from the viewpoint of production of an insulation-isolated electronic device.

The processes described above are the type in which the N-type layer isformed prior to changing into porous structure and then only the P-typesubstrate is selectively changed into porous structure by anodization.

III. The present invention will be described below in detail by way ofexamples. However, the invention is not limited to these examples exceptas defined in the appended claims.

EXAMPLE 1

A porous Si layer 21 was formed to a thickness of 50 μm (t₂=50 μm) onthe entirety of one of the major surfaces of a monocrystalline Sisubstrate 22 by anodization (FIG. 1A).

Anodization was performed under the following conditions:

Applied voltage: 2.6 (V)

Current density: 30 (mA·cm⁻²)

Anodizing solution: HF:H₂O:C₂H₅OH=1:1:1

Time: 0.4 (hours)

Thickness of porous Si: 50 (μm)

Porosity: 56 (%)

Thereafter, the porous Si/monocrystalline Si substrate was subjected toselective etching using a 49%. HF solution. In thirty-three minutes, theporous Si was selectively etched with the monocrystalline Si acting asan etch stopper, only the monocrystalline Si being left behind, as shownin FIG. 1B.

EXAMPLE 2

Prior to anodization, boron ions were implanted in one of the surfacesof a monocrystalline Si substrate 32 at an average concentration of1.0×10¹⁹ cm⁻³ in stripes spaced apart from each other by a distance of100 μm. As shown in FIG. 2A, porous Si 31 was formed by anodization instripes spaced apart from each other by a distance (b₃=100 μm) of 100μm, each stripe having a width (a₃=100 μm) of 100 μm and a thickness(t₃=1 μm) of 1 μm.

Anodization was performed under the following conditions:

Applied voltage: 2.6 (V)

Current density: 30 (mA·cm⁻²)

Anodizing solution: HF:H₂O:C₂H₅OH=1:1:1

Thickness of porous Si: 1 (μm)

Porosity: 56 (%)

Thereafter, the porous Si/monocrystalline Si substrate was subjected toselective etching using a 49% HF solution. In two minutes, the porous Siwas selectively etched, only the monocrystalline Si being left behind,as shown in FIG. 2B.

EXAMPLE 3

A 3 μm (u₄=3 μm) thick polycrystalline Si layer 41 was formed on amonocrystalline Si substrate 42 by CVD (FIG. 3A). As shown in FIG. 3B, asurface layer of 2 μm (t₄=2 μm) of the polycrystalline Si layer 41 wasmade porous by anodization to form a porous Si layer 43.

Anodization was performed under the following conditions:

Applied voltage: 2.6 (V)

Current density: 30 (mA·cm⁻²)

Anodizing solution: HF:H₂O:C₂H₅OH=1:1:1

Thickness of porous Si: 2 (μm)

Porosity: 56 (%)

Thereafter, the porous Si/polycrystalline Si/monocrystalline Sisubstrate was subjected to selective etching using a 49% HF solution. Infour minutes, the porous Si was selectively etched with thepolycrystalline Si acting as an etch stopper, only the polycrystallineSi and monocrystalline Si being left behind, as shown in FIG. 3C.

EXAMPLE 4

A 3 μm (u₅=3 m) thick polycrystalline Si layer 51 was formed on amonocrystalline Si substrate 52 by CVD. Prior to anodization, boron ionswere implanted into the surface of the polycrystalline Si layer 51 at1.0×10¹⁹ cm⁻³ in stripes spaced apart from each other by a distance of20 μm. As shown in FIG. 4A, porous Si 53 was formed by anodization instripes spaced apart from each other by a distance (b₅=20 μm) of 20 μm,each stripe having a width (a₅=20 μm) of 20 μm and a thickness (t₅=1 μm)of 1 μm.

Anodization was performed under the following conditions:

Applied voltage: 2.6 (V)

Current density: 30 (mA·cm⁻²)

Anodizing solution: HF:H₂O:C₂H₅OH=1:1:1

Thickness of porous Si: 1 (μm)

Porosity: 56 (%)

Thereafter, the porous Si/polycrystalline Si/monocrystalline Sisubstrate was subjected to selective etching using a 49% HF solution. Intwo minutes, the porous Si was selectively etched, only thepolycrystalline Si and monocrystalline Si being left behind, as shown inFIG. 4B.

EXAMPLE 5

A porous Si layer 61 was formed to a thickness of 50 μm (t₆=50 μm) onthe entirety of one of the major surfaces of a monocrystalline Sisubstrate 62 by anodization (FIG. 5A).

Anodization was performed under the following conditions:

Applied voltage: 2.6 (V)

Current density: 30 (mA·cm⁻²)

Anodizing solution: HF:H₂O:C₂H₅OH=1:1:1

Time: 0.4 (hour)

Thickness of porous Si: 50 (μm)

Porosity: 56 (%)

As shown in FIG. 5B, a resist 63 was patterned in stripes spaced apartfrom each other by a distance (b₆=100 μm) of 100 μm, each stripe havinga width (a₆=100 μm) of 100 μm.

Thereafter, the porous Si/monocrystalline Si substrate was subjected toselective etching using a 49% HF solution. In thirty-three minutes, theporous Si was selectively removed, only the monocrystalline Si beingleft behind, as shown in FIG. 5C. Finally, the resist was removed (FIG.SD).

EXAMPLE 6

Etching was performed in the same manner as that of Example 1 with theexception that a mixture solution (10:1) of 49% hydrofluoric acid andethyl alcohol was used as an etchant. In twenty-nine minutes afterinitialization of etching, the porous Si was selectively removed withthe monocrystalline Si acting as an etch stopper.

EXAMPLE 7

Etching was performed in the same manner as that of Example 2 with theexception that a mixture solution (10:1) of 49% hydrofluoric acid andethyl alcohol was used as an etchant. In one point seven minutes afterinitialization of etching, the porous Si was selectively removed, onlythe monocrystalline Si being left behind.

EXAMPLE 8

Etching was performed in the same manner as that of Example 3 with theexception that a mixture solution (10:1) of 49% hydrofluoric acid andethyl alcohol was used as an etchant. In three point four minutes afterinitialization of etching, the porous Si was selectively removed withthe polycrystalline Si acting as an etch stopper, only thepolycrystalline Si and the monocrystalline Si being left behind, asshown in FIG. 3C.

EXAMPLE 9

Etching was performed in the same manner as that of Example 4 with theexception that a mixture solution (10:1) of 49% hydrofluoric acid andethyl alcohol was used as an etchant. In one point seven minutes afterinitialization of etching, the porous Si was selectively removed, onlythe polycrystalline Si and monocrystalline Si being left behind, asshown in FIG. 4B.

EXAMPLE 10

Etching was performed in the same manner as that of Example 5 with theexception that a mixture solution (10:1) of 49% hydrofluoric acid andethyl alcohol was used as an etchant. In twenty-nine minutes afterinitialization of etching, the porous Si was selectively removed, onlythe monocrystalline Si being left behind, as shown in FIG. 5C. Finally,.the resist was removed (FIG. 5D).

EXAMPLE 11

Etching was performed in the same manner as that of Example 1 with theexception that a mixture solution (1:5) of 49% hydrofluoric acid andhydrogen peroxide was used as an etchant. In twenty-one minutes afterinitialization of etching, the porous Si was selectively removed withthe monocrystalline Si acting as an etch stopper, only themonocrystalline Si being left behind, as shown in FIG. 1B.

EXAMPLE 12

Etching was performed in the same manner as that of Example 2 with theexception that a mixture solution (1:5) of 49% hydrofluoric acid andhydrogen peroxide was used as an etchant. In one point three minutesafter initialization of etching, the porous Si was selectively removed,only the monocrystalline Si being left behind, as shown in FIG. 2B.

EXAMPLE 13

Etching was performed in the same manner as that of Example 3 with theexception that a mixture solution (1:5) of 49% hydrofluoric acid andhydrogen peroxide was used as an etchant. In two point six minutes afterinitialization of etching, the porous Si was selectively removed withthe polycrystalline Si acting as an etch stopper, only thepolycrystalline Si and the monocrystalline Si being left behind, asshown in FIG. 3C.

EXAMPLE 14

Etching was performed in the same manner as that of Example 4 with theexception that a mixture solution (1:5) of 49% hydrofluoric acid andhydrogen peroxide was used as an etchant. In one point three minutesafter initialization of etching, only the porous Si was selectivelyremoved, only the polycrystalline Si and monocrystalline Si being leftbehind, as shown in FIG. 4B.

EXAMPLE 15

Etching was performed in the same manner as that of Example 5 with theexception that a mixture solution (1:5) of 49% hydrofluoric acid andhydrogen peroxide was used as an etchant. In this etching, only theporous Si was selectively removed, leaving the monocrystalline Sibehind, as shown in FIG. 5C. Finally, the resist was removed (FIG. 5D).

EXAMPLE 16

Etching was performed in the same manner as that of Example 1 with theexception that a mixture solution (10:6:50) of 49% hydrofluoric acid,ethyl alcohol and hydrogen peroxide was used as an etchant. Intwenty-six minutes after initialization of etching, the porous Si wasselectively removed with the remaining monocrystalline Si acting as anetch stopper, as shown in FIG. 1B.

EXAMPLE 17

Etching was performed in the same manner as that of Example 2 with theexception that a mixture solution (10:6:50) of 49% hydrofluoric acid,ethyl alcohol and hydrogen peroxide was used as an etchant. In one pointfour minutes after initialization of etching, the porous Si wasselectively removed, only the monocrystalline Si being left behind, asshown in FIG. 2B.

EXAMPLE 18

Etching was performed in the same manner as that of Example 3 with theexception that a mixture solution (10:6:50) of 49% hydrofluoric acid,ethyl alcohol and hydrogen peroxide was used as an etchant. In two pointeight minutes after initialization of etching, the porous Si wasselectively removed with the polycrystalline Si acting as an etchstopper, only the polycrystalline Si and the monocrystalline Si beingleft behind, as shown in FIG. 3C.

EXAMPLE 19

Etching was performed in the same manner as that of Example 4 with theexception that a mixture solution (10:6:50) of 49% hydrofluoric acid,ethyl alcohol and hydrogen peroxide was used as an etchant. In one pointfour minutes after initialization of etching, the porous Si wasselectively removed, only the polycrystalline Si and monocrystalline Sibeing left behind, as shown in FIG. 4B.

EXAMPLE 20

Etching was performed in the same manner as that of Example 5 with theexception that a mixture solution (10:6:50) of 49% hydrofluoric acid,ethyl alcohol and hydrogen peroxide was used as an etchant. Intwenty-eight minutes after initialization of etching, the porous Si wasselectively removed, only the monocrystalline Si being left behind, asshown in FIG. 5C. Finally, the resist was removed (FIG. 5D).

EXAMPLE 21

Etching was performed in the same manner as that of Example 1 with theexception that a buffered hydrofluoric acid (NH₄F: 36.2%, HF: 4.5%) wasused as an etchant. In nineteen minutes after initialization of etching,the porous Si was selectively removed with the monocrystalline Si actingas an etch stopper, as shown in FIG. 1B.

EXAMPLE 22

Etching was performed in the same manner as that of Example 2 with theexception that a buffered hydrofluoric acid (NH₄F: 36.2%, HF: 4.5%) wasused as an etchant. In seven seconds after initialization of etching,only the porous Si was selectively removed, leaving the monocrystallineSi behind, as shown in FIG. 2B.

EXAMPLE 23

Etching was performed in the same manner as that of Example 3 with theexception that a buffered hydrofluoric acid (NH₄F: 36.2%, HF: 4.5%) wasused as an etchant. In fourteen seconds after initialization of etching,the porous Si was selectively removed with the polycrystalline Si actingas an etch stopper, only the polycrystalline Si and the monocrystallineSi being left behind, as shown in FIG. 3C.

EXAMPLE 24

Etching was performed in the same manner as that of Example 4 with theexception that a buffered hydrofluoric acid (NH₄F: 36.2%, HF: 4.5%) wasused as an etchant. In seven seconds after initialization of etching,only the porous Si was selectively removed, leaving the polycrystallineSi and monocrystalline Si behind, as shown in FIG. 4B.

EXAMPLE 25

Etching was performed in the same manner as that of Example 5 with theexception that a buffered hydrofluoric acid (NH₄F: 36.2%, HF: 4.5%) wasused as an etchant. In nineteen minutes after initialization of etching,the porous Si was selectively removed, only the monocrystalline Si beingleft behind, as shown in FIG. 5C. Finally, the resist was removed (FIG.5D).

EXAMPLE 26

Etching was performed in the same manner as that of Example 1 with theexception that a mixture solution (10:1) of buffered hydrofluoric acidand ethyl alcohol was used as an etchant. In twenty-one minutes afterinitialization of etching, the porous Si was selectively removed withthe monocrystalline Si acting as an etch stopper, only themonocrystalline Si being left behind, as shown in FIG. 1B.

EXAMPLE 27

Etching was performed in the same manner as that of Example 2 with theexception that a mixture solution (10:1) of buffered hydrofluoric acidand ethyl alcohol was used as an etchant. In seven seconds afterinitialization of etching, only the porous Si was selectively removed,leaving the monocrystalline Si behind, as shown in FIG. 2B.

EXAMPLE 28

Etching was performed in the same manner as that of Example 3 with theexception that a mixture solution (10:1) of buffered hydrofluoric acidand ethyl alcohol was used as an etchant. In fourteen seconds afterinitialization of etching, the porous Si was selectively removed withthe polycrystalline Si acting as an etch stopper, only thepolycrystalline Si and the monocrystalline Si being left behind, asshown in FIG. 3C.

EXAMPLE 29

Etching was performed in the same manner as that of Example 4 with theexception that a mixture solution (10:1) of buffered hydrofluoric acidand ethyl alcohol was used as an etchant. In seven seconds afterinitialization of etching, only the porous Si was selectively removed,leaving the polycrystalline Si and monocrystalline Si behind, as shownin FIG. 4B.

EXAMPLE 30

Etching was performed in the same manner as that of Example 5 with theexception that a mixture solution (10:1) of buffered hydrofluoric acidand ethyl alcohol was used as an etchant. In twenty-one minutes afterinitialization of etching, the porous Si was selectively removed, onlythe monocrystalline Si being left behind, as shown in FIG. 5C. Finally,the resist was removed (FIG. 5D).

EXAMPLE 31

Etching was performed in the same manner as that of Example 1 with theexception that a mixture solution (1:5) of buffered hydrofluoric acidand hydrogen peroxide was used as an etchant. In nine minutes afterinitialization of etching, the porous Si was selectively removed withthe monocrystalline Si acting as an etch stopper, only themonocrystalline Si being left behind, as shown in FIG. 1B.

EXAMPLE 32

Etching was performed in the same manner as that of Example 2 with theexception that a mixture solution (1:5) of buffered hydrofluoric acidand hydrogen peroxide was used as an etchant. In five seconds afterinitialization of etching, only the porous Si was selectively removed,leaving the monocrystalline Si behind, as shown in FIG. 2B.

EXAMPLE 33

Etching was performed in the same manner as that of Example 3 with theexception that a mixture solution (1:5) of buffered hydrofluoric acidand hydrogen peroxide was used as an etchant. In ten seconds afterinitialization of etching, the porous Si was selectively removed withthe polycrystalline Si acting as an etch stopper, only thepolycrystalline Si and the monocrystalline Si being left behind, asshown in FIG. 3C.

EXAMPLE 34

Etching was performed in the same manner as that of Example 4 with theexception that a mixture solution (1:5) of buffered hydrofluoric acidand hydrogen peroxide was used as an etchant. In five seconds afterinitialization of etching, only the porous Si was selectively removed,leaving the polycrystalline Si and monocrystalline Si behind, as shownin FIG. 4B.

EXAMPLE 35

Etching was performed in the same manner as that of Example 5 with theexception that a mixture solution (1:5) of buffered hydrofluoric acidand hydrogen peroxide was used as an etchant. In nine minutes, theporous Si was selectively removed, only the monocrystalline Si beingleft behind, as shown in FIG. 5C. Finally, the resist was removed (FIG.5D).

EXAMPLE 36

Etching was performed in the same manner as that of Example 1 with theexception that a mixture solution (10:6:50) of buffered hydrofluoricacid (NH₄F: 36.2%, HF: 4.5%), ethyl alcohol and hydrogen peroxide wasused as an etchant. In ten minutes after initialization of etching, theporous Si was selectively removed with the monocrystalline Si acting asan etch stopper, only the monocrystalline Si being left, as shown inFIG. 1B.

EXAMPLE 37

Etching was performed in the same manner as that of Example 2 with theexception that a mixture solution (10:6:50) of buffered hydrofluoricacid (NH₄F: 36.2%, HF: 4.5%), ethyl alcohol and hydrogen peroxide wasused as an etchant. In six seconds after initialization of etching, onlythe porous Si was selectively removed, leaving the monocrystalline Sibehind, as shown in FIG. 2B.

EXAMPLE 38

Etching was performed in the same manner as that of Example 3 with theexception that a mixture solution (10:6:50) of buffered hydrofluoricacid (NH₄F: 36.2%, HF: 4.5%), ethyl alcohol and hydrogen peroxide wasused as an etchant. In twelve seconds after initialization of etching,the porous Si was selectively removed with the polycrystalline Si actingas an etch stopper, only the polycrystalline Si and the monocrystallineSi being left behind, as shown in FIG. 3C.

EXAMPLE 39

Etching was performed in the same manner as that of Example 4 with theexception that a mixture solution (10:6:50) of buffered hydrofluoricacid (NH₄F: 36.2%, HF: 4.5%), ethyl alcohol and hydrogen peroxide wasused as an etchant. In six seconds after initialization of etching, onlythe porous Si was selectively removed, leaving the polycrystalline Siand monocrystalline Si behind, as shown in FIG. 4B.

EXAMPLE 40

Etching was performed in the same manner as that of Example 5 with theexception that a mixture solution (10:6:50) of buffered hydrofluoricacid (NH₄F: 36.2%, HF: 4.5%), ethyl alcohol and hydrogen peroxide wasused as an etchant. In ten minutes after initialization of etching, theporous Si was selectively removed, only the monocrystalline Si beingleft behind, as shown in FIG. 5C. Finally, the resist was removed (FIG.5D).

EXAMPLE 41

Anodization was conducted on a P-type (100) monocrystalline Si substratehaving a thickness of 200 μm in a 50% HF solution at a current densityof 100 mA/cm². The porous structure formation rate was 8.4 μm/min andhence it took twenty four minutes for the 200 μm-thick P-type (100) Sisubstrate to be made entirely porous.

A Si epitaxial layer with a thickness of of 0.5 μm was grown on theP-type (100) porous Si substrate at a low temperature by molecular beamepitaxy (MBE). Deposition was conducted under the following conditions:

Temperature: 700° C.

Pressure: 1×10⁻⁹ Torr

Growth rate: 0.1 nm/sec.

Next, an optically polished fused silica glass substrate was placed onthe surface of the epitaxial layer. The whole structure was then heatedat 800° C. in an oxygen atmosphere for 0.5 hours to firmly join the twosubstrates to each other.

Si₃N₄ was deposited to a thickness of 0.1 μm by plasma CVD method tocover the bonded substrates, and then only the nitride film on theporous substrate was removed by reactive ion etching. Thereafter,selective etching was conducted on the bonded substrates in a 49%hydrofluoric acid solution while the solution was being stirred. Inseventy eight minutes, the porous Si substrate was completely etchedwith the monocrystalline Si layer acting as an etch stopper, only themonocrystalline Si layer being left behind.

The etching rate of the non-porous monocrystalline Si was so low thatonly a maximum of 50 Å of non-porous monocrystalline Si was removed inseventy eight minutes. Since the ratio of the etching rate of thenon-porous monocrystalline Si to that of the porous layer is 1:10⁵ ormore, the amount of non-porous layer which is etched (several. tensangstroms) can be ignored in a practical operation. That is, the 200μm-thick porous Si substrate was removed, and subsequently the Si₃N₄layer was removed with a result that the 0.5 μm-thick monocrystalline Silayer formed on the glass substrate remained.

The cross-section of the monocrystalline Si layer was observed with atransmission type electron microscope. It was found that no crystaldefect was newly introduced in the Si layer and hence the Si layer hadexcellent crystalline structure.

EXAMPLE 42

Anodization was conducted on a P type (100) monocrystalline Si substratehaving a thickness of 200 μm in a 50% HF solution at a current densityof 100 mA/cm². The porous structure formation rate was 8.4 μm/min andhence it took twenty four minutes for the 200 μm-thick P type (100) Sisubstrate to be made entirely porous. A Si epitaxial layer with athickness of 5.0 μm was grown on the P type (100) porous Si substrate ata low temperature by plasma CVD. Deposition was conducted under thefollowing conditions:

Gas: SiH₄

High-frequency power: 100 W

Temperature: 800° C.

Pressure: 1×10⁻² Torr

Growth rate: 2.5 nm/sec.

Next, an optically polished glass substrate having a softening point ofabout 500° C. was placed on the surface of the epitaxial layer. Thewhole structure was heated at 450° C. in an oxygen atmosphere for 0.5hours to firmly join the two substrates to each other.

Si₃N₄ was deposited to a thickness of 0.1 μm by plasma CVD to cover thetwo bonded substrates, and then only the nitride film on the poroussubstrate was removed by reactive ion etching.

Thereafter, selective etching was conducted on the bonded substrates ina 49% hydrofluoric acid solution while the solution was being stirred.In seventy eight minutes, the porous Si substrate was completely etchedwith the monocrystalline Si layer acting as an etch stopper, only themonocrystalline Si layer being left behind.

The etching rate of the non-porous monocrystalline Si was so low thatonly a maximum of 50 Å of non-porous monocrystalline Si was removed inseventy eight minutes. Since the ratio of the etching rate of thenon-porous monocrystalline Si to that of the porous layer is 1:10⁵ ormore, the amount of non-porous layer which is etched (several tensangstroms) can be ignored in a practical operation. That is, the 200μm-thick porous Si substrate was removed, and subsequently the Si₃N₄layer was removed with a result that the 5.0 μm-thick monocrystalline Silayer formed on the glass substrate having a low softening pointremained.

EXAMPLE 43

Anodization was conducted on a P type (100) monocrystalline Si substratehaving a thickness of 200 μm in a 50% HF solution at a current densityof 100 mA/cm². The porous structure formation rate was 8.4 μm/min andhence it took twenty four minutes for the 200 μm-thick P type (100) Sisubstrate to be made entirely porous. A Si epitaxial layer with athickness of 1.0 μm was grown on the P type (100) porous Si substrate ata low temperature by bias sputtering. Deposition was conducted under thefollowing conditions:

RF frequency: 100 MHz

High-frequency power: 600 W

Temperature: 300° C.

Ar gas pressure: 8×10⁻³ Torr

Growth rate: 120 minutes

Target d.c. bias: −200 V

Substrate d.c. bias: +5 V

Next, an optically polished glass substrate having a softening point ofabout 500° C. was placed on the surface of the epitaxial layer. Thewhole structure was heated at 450° C. in an oxygen atmosphere for 0.5 ofan hour firmly join the two substrates to each other.

Si₃N₄ was deposited to a thickness of 0.1 μm by plasma CVD to cover thetwo bonded substrates, and then only the nitride film on the poroussubstrate was removed by reactive ion etching.

Thereafter, selective etching was conducted on the bonded substrates ina 49% hydrofluoric acid solution while the solution was being stirred.In seventy eight minutes, the porous Si substrate was completely etchedwith the monocrystalline Si layer acting as an etch stopper, only themonocrystalline Si layer being left behind.

The etching rate of the non-porous monocrystalline Si was so low thatonly a maximum of 50 Å of non-porous monocrystalline Si was removed inseventy eight minutes. Since the ratio of the etching rate of thenon-porous monocrystalline Si to that of the porous layer is 1:10⁵ ormore, the amount of non-porous layer which is etched (several tensangstroms) can be ignored in a practical operation. That is, the 200μm-thick porous Si substrate was removed, and subsequently the Si₃N₄layer was removed with a result that the 1.0 μm-thick monocrystalline Silayer on the glass substrate having a low softening point remained.

In case of coating of Apiexon Wax or Electron Wax in place of the Si₃N₄layer, the same effect was obtained and only the Si substrate madeporous was completely removed.

EXAMPLE 44

Anodization was conducted on a N type (100) monocrystalline Si substratehaving a thickness of 200 μm in a 50% HF solution at a current densityof 100 mA/cm². The porous structure formation rate was 8.4 μm/min andhence it took twenty four minutes for the 200 μm-thick N type (100) Sisubstrate to be made entirely porous. A Si epitaxial layer with athickness of 10 μm was grown on the N type (100) porous Si substrate ata low temperature by liquid phase growth method under the followingconditions:

Solvent: Sn,

Solute: Si

Growth temperature: 900° C.

Growth atmosphere: H₂

Growth time: 20 minutes

Next, an optically polished glass substrate having a softening point ofabout 800° C. was placed on the surface of the epitaxial layer. Thewhole structure was heated at 750° C. in an oxygen atmosphere for 0.5hours to firmly join the two substrates to each other.

Si₃N₄ was deposited to a thickness of 0.1 μm by plasma CVD to cover thetwo bonded substrates, and then only the nitride film on the poroussubstrate was removed by reactive ion etching.

Thereafter, selective etching was conducted on the bonded substrates ina 49% hydrofluoric acid solution while the solution was being stirred.In seventy eight minutes, the porous Si substrate was completely etchedwith the monocrystalline Si layer acting as an etch stopper, only themonocrystalline Si layer being left behind.

The etching rate of the non-porous monocrystalline Si was so low thatonly a maximum of 50 Å of non-porous monocrystalline Si was removed inseventy eight minutes. Since the ratio of the etching rate of thenon-porous monocrystalline Si to that of the porous layer is 1:10⁵ ormore, the amount of non-porous layer which is etched (several tensangstroms) can be ignored in a practical operation. That is, the 200μm-thick porous Si substrate was removed, and subsequently the Si₃N₄layer was removed with a result that the 10 μm-thick monocrystalline Silayer on the glass substrate remained.

Coating of Apiezon Wax in place of the Si₃N₄ layer was also effectiveand assured complete removal of only the porous Si substrate.

EXAMPLE 45

Anodization was conducted on a P type (100) monocrystalline Si substratehaving a thickness of 200 μm in a 50% HF solution at a current densityof 100 mA/cm².

The porous structure formation rate was 8.4 μm/min and hence it tooktwenty four minutes for the 200 μm-thick P type (100) Si substrate to bemade entirely porous. A Si epitaxial layer with a thickness of 1.0 μmwas grown on the P type (100) porous Si substrate at a low temperatureby low-pressure CVD. Deposition was conducted under the followingconditions:

Source gas: SiH₄800 SCCM

Carrier gas: H₂ ₁₅₀ liter/min

Temperature: 850° C.

Pressure: 1×10⁻² Torr

Growth rate: 3.3 nm/sec

Next, an optically polished fuzed silica glass substrate was placed onthe surface of the epitaxial layer. The whole structure was heated at800° C. in an oxygen atmosphere for 0.5 hours to firmly join the twosubstrates to each other.

Si₃N₄ was deposited to a thickness of 0.1 μm by plasma CVD to cover thetwo bonded substrates, and then only the nitride film on the poroussubstrate was removed by reactive ion etching.

Thereafter, selective etching was conducted on the bonded substrates ina 49% hydrofluoric acid solution while the solution was being stirred.In seventy eight minutes, the porous Si substrate was completely etchedwith the monocrystalline Si layer acting as an etch stopper, themonocrystalline Si layer being left behind.

The etching rate of the non-porous monocrystalline Si was so low thatonly a maximum of 50 Å of non-porous monocrystalline Si was removed inseventy eight minutes. Since the ratio of the etching rate of thenon-porous monocrystalline Si to that of the porous layer is 1:10⁵ ormore, the amount of non-porous layer which is etched (several tensangstroms) can be ignored in a practical operation. That is, the 200μm-thick porous Si substrate was removed, and subsequently the Si₃N₄layer was removed with a result that the 1.0-thick μm monocrystalline Silayer on the silica glass substrate remained.

When SiH₂Cl₂ was used as the source gas, the growth temperature had tobe higher by several tens of degrees. However, high-speed etchingcharacteristics to the porous substrate did not deteriorate.

EXAMPLE 46

A Si epitaxial layer with a thickness of 1.0 μm was grown on a P type(100) Si substrate having a thickness of 200 μm by CVD. Deposition wasconducted under the following conditions:

Reactive gas flow rate:

SiH₄ 1000 SCCM

H₂ 230 liter/min

Temperature: 1080° C.

Pressure: 760 Torr

Time: 2 min

Anodization was conducted on the substrate in a 50% HF solution at acurrent density of 100 mA/cm². The porous structure formation rate was8.4 μm/min and hence it took twenty four minutes for the 200 μm-thick Ptype (100) Si substrate to be made entirely porous. At that time, therewas no change in the Si epitaxial layer.

Next, an optically polished fuzed silica glass substrate was placed onthe surface of the epitaxial layer. The whole structure was heated at800° C. in an oxygen atmosphere for 0.5 hours to firmly join the twosubstrates to each other.

Si₃N₄ was deposited to a thickness of 0.1 μm by plasma CVD to cover thetwo bonded substrates, and then only the nitride film on the poroussubstrate was removed by reactive ion etching.

Thereafter, selective etching was conducted on the bonded substrates ina 49% hydrofluoric acid solution while the solution was being stirred.In seventy eight minutes, the porous Si substrate was completely removedwith the monocrystalline Si layer acting as an etch stopper, only themonocrystalline Si layer being left behind.

The etching rate of the non-porous monocrystalline Si was so low thatonly a maximum of 50 Å of non-porous monocrystalline Si was removed inseventy eight minutes. Since the ratio of the etching rate of thenon-porous monocrystalline Si to that of the porous layer is 1:10⁵, theamount of non-porous layer which is etched (several tens angstroms) canbe ignored in a practical operation. That is, the 200 μm-thick porous Sisubstrate was removed, and subsequently the Si₃N₄ layer was removed witha result that the 1.0 μm monocrystalline Si layer on the silica glasssubstrate remained.

The cross-section of the monocrystalline Si layer was observed with atransmission type electron microscope. It was found that no crystaldefect was newly introduced in the Si layer and hence the Si layer hadexcellent crystalline structure.

EXAMPLE 47

A Si epitaxial layer with a thickness of 0.5 μm was grown on a P type(100) Si substrate having a thickness of 200 μm by CVD. Deposition wasconducted under the following conditions:

Reactive gas flow rate:

SiH₂Cl₂ 1000 SCCM

H₂ 230 liter/min

Temperature: 1080° C.

Pressure: 80 Torr

Time: 1 min

Anodization was conducted on the substrate in a 50% HF solution at acurrent density of 100 mA/cm². The porous structure formation rate was8.4 μm/min and hence it took twenty four minutes for the 200 μm-thick Ptype (100) Si substrate to be made entirely porous. As mentioned above,anodization made only the P type (100) Si substrate porous, and therewas no change in the Si epitaxial layer.

Next, an optically polished fuzed silica glass substrate was placed onthe surface of the epitaxial layer. The whole structure was heated at800° C. in an oxygen atmosphere for 0.5 hours to firmly join the twosubstrates to each other.

Si₃N₄ was deposited to a thickness of 0.1 μm by plasma CVD to cover thetwo bonded substrates, and then only the nitride film on the poroussubstrate was removed by reactive ion etching.

Thereafter, selective etching was conducted on the bonded substrates ina 49% hydrofluoric acid solution while the solution was being stirred.In seventy eight minutes, the porous Si substrate was completely etchedwith the monocrystalline Si layer acting as an etch stopper, only themonocrystalline Si layer being left behind.

The etching rate of the non-porous monocrystalline Si was so low thatonly a maximum of 50 Å of non-porous monocrystalline Si was removed inseventy eight minutes. Since the ratio of the etching rate of thenon-porous monocrystalline Si to that of the porous layer is 1:10⁵, theamount of non-porous layer which is etched (several tens angstroms) canbe ignored in a practical operation. That is, the 200 μm-thick porous Sisubstrate was removed, and subsequently the Si₃N₄ layer was removed witha result that the 0.5 μm-thick monocrystalline Si layer on the silicaglass substrate remained.

The cross-section of the monocrystalline Si layer was observed by atransmission type electron microscope. It was found that no crystaldefect was newly introduced in the Si layer and hence the Si layer hadexcellent crystalline structure.

EXAMPLE 48

A N type Si layer with a thickness of 1 μm was formed on a P type (100)Si substrate having a thickness of 200 μm by proton implantation.Implantation rate of H⁺ was 5×10¹⁵ (ions/cm²).

Anodization was conducted on the substrate in a 50% HF solution at acurrent density of 100 mA/cm². The porous structure formation rate was8.4 μm/min and hence it took twenty four minutes for the 200 μm-thick Ptype (100) Si substrate to be made entirely porous. As mentioned above,anodization made only the P type (100) Si substrate porous, and therewas no change in the N type Si layer.

Next, an optically polished fuzed silica glass substrate was placed onthe surface of the N type Si layer. The whole structure was heated at800° C. in an oxygen atmosphere for 0.5 hours to firmly join the twosubstrates to each other.

Si₃N₄ was deposited to a thickness of 0.1 μm by plasma CVD to cover thetwo bonded substrates, and then only the nitride film on the poroussubstrate was removed by reactive ion etching.

Thereafter, selective etching was conducted on the bonded substrates ina 49% hydrofluoric acid solution while the solution was being stirred.In seventy eight minutes, the porous Si substrate was completely etchedwith the monocrystalline Si layer acting as an etch stopper, only themonocrystalline Si layer being left behind.

The etching rate of the non-porous monocrystalline Si was so low thatonly a maximum of 50 Å of non-porous monocrystalline Si was removed inseventy eight minutes. Since the ratio of the etching rate of thenon-porous monocrystalline Si to that of the porous layer is 1:10⁵, theamount of non-porous layer which is etched (several tens angstroms) canbe ignored in a practical operation. That is, the 200 μm-thick porous Sisubstrate was removed, and subsequently the Si₃N₄ layer was removed witha result that the 1 μm-thick monocrystalline Si layer on the silicaglass substrate remained.

The cross-section of the monocrystalline Si layer was observed by atransmission type electron microscope. It was found that no crystaldefect was newly introduced in the Si layer and hence the Si layer hadexcellent crystalline structure.

EXAMPLE 49

Anodization was conducted on a P type (100) monocrystalline Si substratehaving a thickness of 200 μm in a HF solution under the followingconditions:

Applied voltage: 2.6 (V)

Current density: 30 (mA·cm⁻²)

Anodizing solution: HF:H₂O:C₂H₅OH=1:1:1

Time: 1.6 (hour)

Thickness of porous Si: 200 (μm)

Porosity: 56 (%)

A Si epitaxial layer with a thickness of 0.5 μm was grown on the P type(100) porous Si substrate at a low temperature by molecular beam epitaxy(MBE). Deposition was conducted under the following conditions:

Temperature: 700° C.

Pressure: 1×10⁻⁹ Torr

Growth rate: 0.1 nm/sec.

Next, a second Si substrate with a 5000 Å thick oxidized layer formed onthe surface thereof was placed on the surface of the epitaxial layer.The whole structure was heated at 800° C. in an oxygen atmosphere for0.5 hours to firmly join the two substrates to each other.

Thereafter, selective etching was conducted on the bonded substrates ina 49% hydrofluoric acid solution while the solution was being stirred.In sixty two minutes, the porous Si substrate was completely etched withthe monocrystalline Si layer acting as an etch stopper, only themonocrystalline Si layer being left behind.

The etching rate of the non-porous monocrystalline Si was so low thatonly a maximum of 50 Å of non-porous monocrystalline Si was removed insixty-two minutes. Since the ratio of the etching rate of the non-porousmonocrystalline Si to that of the porous layer is 1:10⁵, the amount ofnon-porous layer which is etched (several tens angstroms) can be ignoredin a practical operation. That is, the 200 μm-thick porous Si substratewas removed with a result that the 0.5 μm-thick monocrystalline Si layerformed on the SiO₂ layer remained.

The cross-section of the monocrystalline Si layer was observed by atransmission type electron microscope. It was found that no crystaldefect was newly introduced in the Si layer and hence the Si layer hadexcellent crystalline structure.

EXAMPLE 50

Anodization was conducted on a P type (100) monocrystalline Si substratehaving a thickness of 200 μm in a HF solution in the same manner as thatof Example 49.

A Si epitaxial layer with a thickness of 0.5 μm was grown on the P type(100) porous Si substrate at a low temperature by plasma CVD. Depositionwas conducted under the following conditions:

Gas: SiH₄

High-frequency power: 100 W

Temperature: 800° C.

Pressure: 1×10⁻² Torr

Growth rate: 2.5 nm/sec.

Next, a second Si substrate with a 5000 Å thick oxidized layer formed onthe surface thereof was placed on the surface of the epitaxial layer.The whole structure was heated at 800° C. in an oxygen atmosphere for0.5 hours to firmly join the two substrates to each other.

Thereafter, selective etching was conducted on the bonded substrates. ina 49% hydrofluoric acid solution while the solution was being stirred.In sixty two minutes, the porous Si substrate was completely etched withthe monocrystalline Si layer acting as an etch stopper, only themonocrystalline Si layer being left behind.

The etching rate of the non-porous monocrystalline Si was so low thatonly a maximum of 50 Å of non-porous monocrystalline Si was removed insixty-two minutes. Since the ratio of the etching rate of the non-porousmonocrystalline Si to that of the porous layer is 1:10⁵, the amount ofnon-porous layer which is etched (several tens angstroms) can be ignoredin a practical operation. That is, the 200 μm-thick porous Si substratewas removed with a result that the 0.5 μm thick monocrystalline Si layeron the SiO₂ layer remained.

EXAMPLE 51

Anodization was conducted on a P type (100) monocrystalline Si substratehaving a thickness of 200 μm in a HF solution in the same manner as thatof Example 49.

A Si epitaxial layer with a thickness of 0.5 μm was grown on the P type(100) porous Si substrate at a low temperature by bias sputtering.Deposition was conducted under the following conditions:

RF frequency: 100 MHz

High-frequency power: 600 W

Temperature: 300° C.

Ar gas pressure: 8×10⁻³ Torr

Growth time: 60 minutes

Target d.c. bias: −200 V

Substrate d.c. bias: +5 V

Next, a second Si substrate with a 5000 Å thick oxidized layer formed onthe surface thereof was placed on the surface of the epitaxial layer.The whole structure was heated at 800° C. in an oxygen atmosphere for0.5 hours to firmly join the two substrates to each other.

Thereafter, selective etching was conducted on the bonded substrates ina 49% hydrofluoric acid solution while the solution was being stirred.In sixty two minutes, the porous Si substrate was completely etched withthe monocrystalline Si layer acting as an etch stopper, themonocrystalline Si layer being left behind.

The etching rate of the non-porous monocrystalline Si was so low thatonly a maximum of 50 Å of non-porous monocrystalline Si was removed insixty-two minutes. Since the ratio of the etching rate of the non-porousmonocrystalline Si to that of the porous layer is 1:10⁵ the amount ofnon-porous layer which is etched (several tens angstroms)-can be ignoredin a practical operation. That is, the 200 μm-thick porous Si substratewas removed with a result that the 0.5 μm thick monocrystalline Si layeron the SiO₂ layer remained.

EXAMPLE 52

Anodization was conducted on a N type (100) monocrystalline Si substratehaving a thickness of 200 μm in a HF solution in the same manner as thatof Example 49.

A Si epitaxial layer with a thickness of 5 μm was grown on the N type(100) porous Si substrate at a low temperature by liquid phase growthunder the following conditions:

Solvent: Sn,

Solute: Si

Growth temperature: 900° C.

Growth atmosphere: H₂

Growth rate: 10 minutes

Next, a second Si substrate with a 5000 Å thick oxidized layer formed onthe surface thereof was placed on the surface of the epitaxial layer.The whole structure was heated at 800° C. in an oxygen atmosphere for0.5 hours to firmly join the two substrates to each other.

Thereafter, selective etching was conducted on the bonded substrates ina 49% hydrofluoric acid solution while the solution was being stirred.In sixty two minutes, the porous Si substrate was completely etched withthe monocrystalline Si layer acting as an etch stopper, themonocrystalline Si layer being left behind.

The etching rate of the non-porous monocrystalline Si was so low thatonly a maximum of 50 Å of non-porous monocrystalline Si was removed insixty-two minutes. Since the ratio of the etching rate of the non-porousmonocrystalline Si to that of the porous layer is 1:10⁵, the amount ofnon-porous layer which is etched (several tens angstroms) can be ignoredin a practical operation. That is, the 200 μm-thick porous Si substratewas removed with a result that the 5 μm thick monocrystalline Si layeron the SiO₂ layer remained.

EXAMPLE 53

Anodization was conducted on a P type (100) monocrystalline Si substratehaving a thickness of 200 μm in a HF solution in the same manner as thatof Example 49.

A Si epitaxial layer with a thickness of 1.0 μm was grown on the P type(100) porous Si substrate at a low temperature by low-pressure CVD.Deposition was conducted under the following conditions:

Source gas: SiH₄

Carrier gas: H₂

Temperature: 850° C.

Pressure: .1×10⁻² Torr

Growth rate: 3.3 nm/sec

Next, a second Si substrate with a 5000 Å thick oxidized layer formed onthe surface thereof was placed on the surface of the epitaxial layer.The whole structure was heated at 800° C. in an oxygen atmosphere for0.5 hours to firmly join the two substrates to each other.

Thereafter, selective etching was conducted on the bonded substrates ina 49% hydrofluoric acid solution while the solution was being stirred.In sixty two minutes, the porous Si substrate was completely etched withthe monocrystalline Si layer acting as an etch stopper, themonocrystalline Si layer being left behind.

The etching rate of the non-porous monocrystalline Si was so low thatonly a maximum of 50 Å of non-porous monocrystalline Si was removed insixty-two minutes. Since the ratio of the etching rate of the non-porousmonocrystalline Si to that of the porous layer is 1:10⁵, the amount ofnon-porous layer which is etched (several tens angstroms) can be ignoredin a practical operation. That is, the 200 μ-thick porous Si substratewas removed with a result that the 1.0 μm thick monocrystalline Si layeron the SiO₂ layer remained.

When SiH₂Cl₂ was used as the source gas, the growth temperature had tobe higher by several tens of degrees. However, high-speed etchingcharacteristics to the porous substrate did not deteriorate.

EXAMPLE 54

A Si epitaxial layer with a thickness of 1 μm was grown on a P type(100) Si substrate having a thickness of 200 μm by low-pressure CVD.Deposition was conducted under the following conditions:

Reactive gas flow rate:

SiH₂Cl₂ 1000 SCCM

H₂ 230 liter/min

Temperature: 1080° C.

Pressure: 80 Torr

Time: 2 min

Anodization was conducted on the substrate in a 50% HF solution at acurrent density of 100 mA/cm². The porous structure formation rate was8.4 μm/min and hence it took twenty four minutes for the 200 μm-thick Ptype (100) Si substrate to be made entirely porous. As mentioned above,anodization made only the P type (100) Si substrate porous, and did notaffect the Si epitaxial layer at all.

Next, a second Si substrate with a 5000 Å-thick oxidized layer formed onthe surface thereof was placed on the surface of the epitaxial layer.The whole structure was heated at 800° C. in an oxygen atmosphere for0.5 hours to firmly join the two substrates to each other.

Thereafter, selective etching was conducted on the bonded substrates ina 49% hydrofluoric acid solution while the solution was being stirred.In sixty two minutes, the porous Si substrate was completely etched withthe monocrystalline Si layer acting as an etch stopper, while themonocrystalline Si layer remained.

The etching rate of the non-porous monocrystalline Si was so low thatonly a maximum of 50 Å of non-porous monocrystalline Si was removed insixty two minutes. Since the ratio of the etching rate of the non-porousmonocrystalline Si to that of the porous layer is 1:10⁵, the amount ofnon-porous layer which is etched (several tens angstroms) can be ignoredin a practical operation. That is, the 200 μm-thick porous Si substratewas removed with a result that the 1.0 μm thick monocrystalline Si layeron the SiO₂ layer remained.

The cross-section of the monocrystalline Si layer was observed by atransmission type electron microscope. It was found that no crystaldefect was newly introduced in the Si layer and hence the Si layer hadexcellent crystalline structure.

EXAMPLE 55

A Si epitaxial layer with a thickness of 5 μm was grown on a P type(100) Si substrate having a thickness of 200 μm by atmospheric CVD.Deposition was conducted under the following conditions:

Reactive gas flow rate:

SiH₂Cl₂ 1000 SCCM

H₂ 230 liter/min

Temperature: 1080° C.

Pressure: 760 Torr

Time: 1 min

Anodization was conducted on the substrate in a HF solution in the samemanner as that of Example 49. As mentioned above, anodization made onlythe P type (100) Si substrate porous, and did not affect the Siepitaxial layer at all.

Next, a second Si substrate with a 5000 Å-thick oxidized layer formed onthe surface thereof was placed on the surface of the epitaxial layer.The whole structure was heated at 800° C. in an oxygen atmosphere for0.5 hours to firmly join the two substrates to each other.

Thereafter, selective etching was conducted on the bonded substrates ina 49% hydrofluoric acid solution while the solution was being stirred.In sixty two minutes, the porous Si substrate was completely etched withthe monocrystalline Si layer acting as an etch stopper, themonocrystalline Si layer being left behind.

The etching rate of the non-porous monocrystalline Si was so low thatonly a maximum of 50 Å of non-porous monocrystalline Si was removed insixty two minutes. Since the ratio of the etching rate of the non-porousmonocrystalline Si to that of the porous layer is 1:10⁵, the amount ofnon-porous layer which is etched (several tens angstroms) can be ignoredin a practical operation. That is, the 200 μm-thick porous Si substratewas removed with a result that the 5.0 μm thick monocrystalline Si layeron the SiO₂ layer remained.

The cross-section of the monocrystalline Si layer was observed by atransmission type electron microscope. It was found that no crystaldefect was newly introduced in the Si layer and hence the Si layer hadexcellent crystalline structure.

EXAMPLE 56

A N type Si layer with a thickness of 1 μm was formed on a P type (100)Si substrate having a thickness of 200 μm by proton implantation.Implantation rate of H⁺ was 5×10¹⁵ (ions/cm²).

Anodization was conducted on the substrate in a 50% HF solution at acurrent density of 100 mA/cm². The porous structure formation rate was8.4 μm/min and hence it took twenty four minutes for the 200 μm-thick Ptype (100) Si substrate to be made entirely porous. As mentioned above,anodization made only the P type (100) Si substrate porous, and did notchange the N type Si layer.

Next, a second Si substrate with a 5000 Å-thick oxidized layer formed onthe surface thereof was placed on the surface of the N type Si layer.The whole structure was heated at 800° C. in an oxygen atmosphere for0.5 hours to firmly join the two substrates to each other.

Thereafter, selective etching was conducted on the bonded substrates ina 49% hydrofluoric acid solution while the solution was being stirred.In sixty two minutes, the porous Si substrate was completely etched withthe monocrystalline Si layer acting as an etch stopper, themonocrystalline Si layer being left behind.

The etching rate of the non-porous monocrystalline Si was so low thatonly a maximum of 50 Å of non-porous monocrystalline Si was removed insixty two minutes. Since the ratio of the etching rate of the non-porousmonocrystalline Si to that of the porous layer is 1:10⁵, the amount ofnon-porous layer which is etched (several tens angstroms) can be ignoredin a practical operation. That is, the 200 μm-thick porous Si substratewas removed with a result that the 1.0 μm thick monocrystalline Si layeron the SiO₂ layer remained.

The cross-section of the monocrystalline Si layer was observed by atransmission type electron microscope. It was found that no crystaldefect was newly introduced in the Si layer and hence the Si layer hadexcellent crystalline structure.

EXAMPLE 57

A P-type (100) single-crystals (monocrystal) Si substrate of a thicknessof 200 μ was anodized in 50% HF solution. The current density then was100 mA/cm^(2.) The porous structure formation rate then was about 8.4μm/min., and the P-type (100) Si substrate of a thickness of 200 μ wasrendered porous in its entirety for 24 minutes.

According to MBE (molecular beam epitaxy) method, an Si epitaxial layerof 0.5 μ was grown at a lower temperature on the P-type (100) porous Sisubstrate. The conditions for deposition are as follows;

temperature: 700° C.

pressure: 1×10⁻⁹ Torr

growth rate: 0.1 nm/sec.

Subsequently, the surface of the epitaxial layer was thermally oxidizedin a depth of 50 nm. A substrate of fused silica glass processed withoptical polishing was bonded onto the thermally oxidized membrane (i.e.,film), and both of the substrates were strongly bonded together byheating at 800° C. for 0.5 hour in oxygen atmosphere.

According to low pressure CVD, Si₃N₄ was deposited to 0.1 μm, therebycoating the bonded two substrates. Thereafter, only the nitride membrane(film) on the porous substrate was removed by reactive ion etching.

Then, the bonded substrates were selectively etched with 49%hydrofluoric acid. In 78 minutes, only the single-crystal Si layerremained without etching, while the porous Si substrate was selectivelyetched with the single-crystal Si as a material for etching stopper andthen completely removed.

The etching rate of the non-porous Si single-crystal (monocrystal) withthe etching. solution was extremely low, such as 50 angstroms or lesseven 78 minutes later, so that the selective ratio of the etching rateof the porous layer to that of the non-porous Si single-crystal was aslarge as 10⁵ or more. The etched amount in the non-porous layer (severaltens angstroms) is a practically negligible decrease in membranethickness. That is, the Si substrate of a thickness of 200 μ, renderedporous, was removed, and after the removal of the Si₃N₄ layer, asingle-crystal Si layer of a thickness of 0.5 μm was formed on thesubstrate of the silica glass.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLE 58

A P-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in 50% HF solution. The current density then was 100 mA/cm².The porous structure formation rate then was about 8.4 μm/min., and theP-type (100) Si substrate of a thickness of 200 μ was rendered porous inits entirety for 24 minutes. According to plasma CVD method, an Siepitaxial layer of 5 μ was grown at a lower temperature on the P-type(100) porous Si substrate. The conditions for-deposition are as follows;

gas: SiH₄

high-frequency power: 100 W

temperature 800° C.

pressure: 1×10⁻² Torr

growth rate: 2.5 nm/sec.

Subsequently, the surface of the epitaxial layer was thermally oxidizedin a depth of 50 nm. A glass substrate, having being processed withoptical polishing and having a softening point around 500° C., wasbonded onto the thermally oxidized membrane, and both of the substrateswere strongly bonded together by heating at 450° C. for 0.5 hour inoxygen atmosphere.

According to plasma CVD method, Si₃N₄ was deposited to 0.1 μm, therebycoating the bonded two substrates. Thereafter, only the nitride membraneon the porous substrate was removed by reactive ion etching.

Then, the bonded substrates were selectively etched with 49%hydrofluoric acid. In 78 minutes, only the single-crystal Si layerremained without etching, while the porous Si substrate was selectivelyetched with the single-crystal Si as a material for etching stopper andthen completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 78 minuteslater, so that the selective ratio of the ethcing rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in the non-porous layer (several tens angstroms)is a practically negligible decrease in membrane thickness. That is, theSi substrate of a thickness of 200 μ, rendered porous, was removed, andafter the removal of the Si₃N₄ layer, a single-crystal Si layer of athickness of 5 μm was formed on the glass substrate of a lower softeningpoint.

EXAMPLE 59

A P-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in 50% HF solution. The current density then was 100 mA/cm².The porous structure formation rate then was about 8.4 μm/min., and theP-type (100) Si substrate of a thickness of 200 μ was rendered porous inits entirety for 24 minutes. According to thermal CVD method, an Siepitaxial layer of 5 μ was grown at a lower temperature on the P-type(100) porous Si substrate. The conditions for deposition are as follows;

gas: SiH₄(0.6 l/min), H₂ (100 l/min)

temperature: 850° C.

pressure: 50 Torr

growth rate: 0.1 μm/min.

Subsequently, the surface of the epitaxial layer was thermally oxidizedin a depth of 50 nm. A glass substrate, having being processed withoptical polishing and having a softening point around 500° C., wasbonded onto the thermally oxidized membrane, and both of the substrateswere strongly bonded together by heating at 450° C. for 0.5 hour inoxygen atmosphere.

According to plasma CVD method, Si₃N₄ was deposited to 0.1 μm, therebycoating the bonded two substrates. Thereafter, only the nitride membraneon the porous substrate was removed by reactive ion etching.

Then, the bonded substrates were selectively etched with 49%hydrofluoric acid. In 78 minutes, only the single-crystal Si layerremained without etching, while the porous Si substrate was selectivelyetched with the single-crystal Si as a material for etching stopper andthen completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 78 minuteslater, so that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in the non-porous layer (several tens angstroms)is a practically negligible decrease in membrane thickness. That is, theSi substrate of a thickness of 200 μ, rendered porous, was removed, andafter the removal of the Si₃N₄ layer, a single-crystal Si layer of athickness of 5 μm was formed on the glass substrate of a lower softeningpoint.

A similar effect could be obtained by using Apiezon wax or electron wax,instead of Si₃N₄, so that only the Si substrate rendered porous could beremoved completely.

EXAMPLE 60

A P-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in 50% HF solution. The current density then was 100 mA/cm².The porous structure formation rate then was about 8.4 μm/min., and theP-type (100) Si substrate of a thickness of 200 μ was rendered porous inits entirety for 24 minutes. According to bias sputter method, an Siepitaxial layer of 1.0 μ was grown at a lower temperature on the P-type(100) porous Si substrate. The conditions for deposition are as follows;

RF frequency: 100 MHz

high-frequency power: 600 W

temperature: 300° C.

Ar gas pressure: 8×10³ Torr

growth time: 120 minutes

target direct current bias: −200 V substrate direct current bias: +5 V.

Subsequently, the surface of the epitaxial layer was thermally oxidizedin a depth of 50 nm. A glass substrate, having being processed withoptical polishing and having a softening point around 500° C., wasbonded onto the thermally oxidized membrane, and both of the substratewere strongly bonded together by heating at 450° C. for 0.5 hour inoxygen atmosphere.

According to plasma CVD method, Si₃N₄ was deposited to 0.1 μm, therebycoating the bonded two substrates. Thereafter, only the nitride membraneon the porous substrate was removed by reactive ion etching.

Then, the bonded substrates were selectively etched with 49%hydrofluoric acid. In 78 minutes, only the single-crystal Si layerremained without etching, while the porous Si substrate was selectivelyetched with the single-crystal Si as a material for etching stopper andthen completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 78 minuteslater, so that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in the non-porous layer (several tens angstroms)is a practically negligible decrease in membrane thickness. That is, theSi substrate of thickness of 200 μ, rendered porous, was removed, andafter the removal of the Si₃N₄ layer, a single-crystal Si layer of athickness of 1.0 μm was formed on the glass substrate of a lower meltingpoint.

A similar effect could be obtained by using Apiezon wax or electron wax,instead of Si₃N₄, so that only the Si substrate rendered porous could beremoved completely.

EXAMPLE 61

A N-type (100 single-crystal Si substrate of a thickness of 200 μ wasanodized in 50% HF solution. The current density then was 100 mA/cm².The porous structure formation rate then was about 8.4 μm/min., and theN-type (100) Si substrate of a thickness of 200 μ was rendered porous inits entirety for 24 minutes. According to liquid phase growth method, anSi epitaxial layer of 10 μ was grown at a lower temperature on theN-type (100) porous Si substrate. The conditions for deposition are asfollows;

solvent: Sn,

Solute: Si

growth temperature: 900° C.

growth atmosphere: H₂

growth time: 20 minutes.

Subsequently, the surface of the epitaxial layer was thermally oxidizedin a depth of 50 nm. A glass substrate, having being processed withoptical polishing and having a softening point around 800° C., wasbonded onto the thermally oxidized membrane, and both of the substrateswere strongly bonded together by heating at 750° C. for 0.5 hour inoxygen atmosphere.

According to low pressure CVD method, Si₃N₄ was deposited to 0.1 μm,thereby coating the bonded two substrates. Thereafter, only the nitridemembrane on the porous substrate was removed by reactive ion etching.

Then, the bonded substrates were selectively etched with 49%hydrofluoric acid. In 78 minutes, only the single-crystal Si layerremained without etching in 78 minutes, while the porous Si substratewas selectively etched with the single-crystal Si as a material foretching stopper and then completely removed.

The etching rate of the non-porous Si-single crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 78 minuteslater, so that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in the non-porous layer (several tens angstroms)is a practically negligible decrease in membrane thickness. That is, theSi substrate, rendered porous, of a thickness of 200 μ was removed, andafter the removal of the Si₃N₄ layer, a single-crystal Si layer of athickness of 10 μm was formed on the glass substrate of a lowersoftening point.

A similar effect could be obtained by using Apiezon wax or electron wax,instead of Si₃N₄, so that only the Si substrate rendered porous could beremoved completely.

EXAMPLE 62

According to CVD method, an Si epitaxial layer of 0.5 μ was grown at alower temperature on a P-type (100) Si substrate of a thickness of 200μ. The conditions for deposition are as follows;

reactive gas flow rate:

SiH₂Cl₂ 1000 SCCM

H₂ 230 1/min.

temperature: 1080° C.

pressure: 80 Torr

time: 1 minute.

The present substrate was anodized in 50% HF solution. The currentdensity then was 100 mA/cm². The porous structure formation rate thenwas about 8.4 μm/min., and the P-type (100) Si substrate of a thicknessof 200 μ was rendered porous in its entirety for 24 minutes. As has beendescribed above, the present anodization rendered only the P-type (100)Si substrate porous, but no change was observed in the Si epitaxiallayer.

Subsequently, the surface of the epitaxial layer was thermally oxidizedin a depth of 50 nm. A substrate of fused silica glass, processed withoptical polishing, was bonded onto the thermally oxidized membrane, andboth of the substrates were strongly bonded together by heating at 800°C. for 0.5 hour in oxygen atmosphere.

According to low pressure CVD method, Si₃N₄ was deposited to 0.1 μm,thereby coating the bonded two substrates. Thereafter, only the nitridemembrane on the porous substrate was removed by reactive ion etching.Then, the bonded substrates were selectively etched with 49%hydrofluoric acid. In 78 minutes, only the single-crystal Si layerremained without etching, while the porous Si substrate was selectivelyetched with the single-crystal Si as a material for etching stopper andthen completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 78 minuteslater, so that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in the non-porous layer (several tens angstroms)is a practically negligible decrease in membrane thickness. That is, theSi substrate of a thickness of 200 μ, rendered porous, was removed, andafter the removal of the Si₃N₄ layer, a single-crystal Si layer of athickness of 0.5 μm was formed on the glass substrate.

A similar effect could be obtained by using Apiezon wax or electron wax,instead of Si₃N₄, so that only the Si substrate rendered porous could beremoved completely.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLE 63

A N-type Si layer 1 μ was formed on the surface of a P-type (100) Sisubstrate of a thickness of 200 μ, by ion implantation of proton. Theimplanted amount of H⁺ was 5×10¹⁵ ions/cm². The substrate was anodizedin 50% HF solution. The current density then was 100 mA/cm^(2.) Theporous structure formation rate then was about 8.4 μm/min., and theP-type (100) Si substrate of a thickness of 200 μ was rendered porous inits entirety for 24 minutes. According to the present anodization as hasbeen described above, only the P-type (100) Si substrate was renderedporous, but no change was observed in the N-type Si layer. Subsequently,the surface of the N-type single-crystal layer was thermally oxidized ina depth of 50 nm. A substrate of fused silica glass processed withoptical polishing was bonded onto the thermally oxidized membrane, andboth of the substrates were strongly bonded together by heating at 800°C. for 0.5 hour in oxygen atmosphere.

According to low pressure CVD method, Si₃N₄ was deposited to 0.1 μm,thereby coating the bonded two substrates. Thereafter, only the nitridemembrane on the porous substrate was removed by reactive ion etching.Then, the bonded substrates were selectively etched with 49%hydrofluoric acid. In 78 minutes, only the single-crystal Si layerremained without etching, while the porous Si substrate was selectivelyetched with the single-crystal Si as a material for etching stopper andthen completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 78 minuteslater, so that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in non-porous layer (several tens angstroms) isa practically negligible decrease in membrane thickness. That is, the Sisubstrate of a thickness of 200 μ, rendered porous, was removed, andafter the removal of the Si₃N₄ layer, a single-crystal Si layer of athickness of 1.0 μm was formed on the glass substrate.

A similar effect could be obtained by using Apiezon wax or electron wax,instead of Si₃N₄, so that only the Si substrate rendered porous could beremoved completely.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLE 64

A P-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in 50% HF solution.

The conditions for deposition are as follows;

applied voltage: 2.6 V

current density: 30 mA·cm⁻²

solution for anodization: HF:H₂O:C₂H₅OH=1:1:1

time: 1.6 hours

thickness of porous Si: 200 μm

porosity: 56%.

According to MBE method, an Si epitaxial layer of 0.5 μ was grown at alower temperature on the P-type (100) porous Si substrate. Theconditions for deposition are as follows;

temperature: 700° C.

pressure: 1×10⁻⁹

growth rate: 0.1 nm/sec.

Subsequently, an oxidized layer of 1000 angstroms was formed on thesurface of the epitaxial layer, and another Si substrate on the surfaceof which was formed an oxidized layer of 5000 angstroms, was bonded tothe oxidized surface. By heating in oxygen atmosphere at 800° C. for 0.5hour, both of the Si substrates were strongly bonded together.

Then, the bonded substrates were selectively etched with 49 hydrofluoricacid while under stirring. In 78 minutes, only the single-crystal Silayer remained without etching, while the porous Si substrate wasselectively etched with the single-crystal Si as a material for etchingstopper and then completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 78 minuteslater, so that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in the non-porous layer (several tens angstroms)is a practically negligible decrease in membrane thickness. That is, theSi substrate of a thickness of 200 μ, rendered porous, was removed, andafter the removal of the Si₃N₄ layer, a single-crystal Si layer of athickness of 0.5 μm was formed on the SiO₂. As a result of observationof the section under a transmission-type electron microscope, it wasconfirmed that no new crystal defect was introduced in the Si layer andthat excellent crystallinity was maintained.

EXAMPLE 65

A P-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in HF solution.

The conditions for anodization are as follows;

applied voltage: 2.6 V

current density: 30 mA·cm⁻²

solution for anodization: HF:H₂O:C₂H₅OH=1:1:1

time: 1.6 hours

thickness of porous Si: 200 μm

porosity: 56%.

According to plasma CVD method, and Si epitaxial layer of 0.5 μ wasgrown at a lower temperature on the P-type (100) porous Si substrate.The conditions for deposition are as follows;

gas: SiH₄

high-frequency power: 100 W

temperature: 800° C.

pressure: 1×10⁻² Torr

growth rate: 2.5 nm/sec.

Subsequently, an oxidized layer of 1000 angstroms was formed on thesurface of the epitaxial layer, and another Si substrate on the surfaceof which was formed as oxidized layer of 5000 angstroms was bonded tothe oxidized surface. By heating in oxygen atmosphere at 800° C. for 0.5hour, both of the Si substrates were strongly bonded together.

Then, the bonded substrates were selectively etched with 49%hydrofluoric acid while under stirring. In 78 minutes, only thesingle-crystal Si layer remained without etching, while the porous Sisubstrate was selectively etched with the single-crystal Si as amaterial for etching stopper and then completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 78 minuteslater, so. that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in the non-porous layer (several tens angstroms)is a practically negligible decrease in membrane thickness. That is, theSi substrate of a thickness of 200 μ, rendered porous, was removed, anda single-crystal Si layer of a thickness of 0.5 μm was formed on theSiO₂.

EXAMPLE 66

A P-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in HF solution.

The conditions for anodization are as follows;

applied voltage: 2.6 V

current density: 30 mA·cm⁻²

solution for anodization: HF:H₂O:C₂H₅OH=1:1:1

time: 1.6 hours

thickness of porous Si: 200 μm

porosity: 56%.

According to bias sputter method, an Si epitaxial layer of 0.5 μ wasgrown at a lower temperature on the P-type (100) porous Si substrate.The conditions for deposition are as follows;

RF frequency: 100 MHz

high-frequency power: 600 W

temperature: 300° C.

Ar gas pressure: 8×10⁻³ Torr

growth time: 60 minutes

target direct current bias: −200 V

substrate direct current bias: +5 V.

Subsequently, an oxidized layer of 1000 angstroms was formed on thesurface of the epitaxial layer, and another Si substrate on the surfaceof which was formed an oxidized layer of 5000 angstroms was bonded tothe oxidized surface. By heating in oxygen atmosphere at 800° C. for 0.5hour, both of the Si substrates were strongly bonded together.

Then, the bonded substrates were selectively etched with 49%hydrofluoric acis while under stirring. In 78 minutes, only thesingle-crystal Si layer remained without etching, while the porous Sisubstrate was selectively etched with the single-crystal Si as amaterial for etching stopper and then completely removed. The etchingrate of the non-porous Si single-crystal with the etching solution wasextremely low, such as 50 angstroms or less even 78 minutes later, sothat the selective ratio of the etching rate of the porous layer to thatof the non-porous Si single-crystal was as large as 10⁵ or more. Theetched amount in the non-porous layer (several tens angstroms) is apractically negligible decrease in membrane thickness. That is, the Sisubstrate of a thickness of 200 μ, rendered porous, was removed, and asingle-crystal Si layer of a thickness of −0.5 μm was formed on theSiO₂.

EXAMPLE 67

A N-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in HF solution.

The conditions for anodization are as follows;

applied voltage: 2.6 V

current density: 30 mA·cm⁻²

solution for anodization: HF:H₂O:C₂H₅OH=1:1:1

time: 1.6 hours

thickness of porous Si: 200 μm

porosity: 56%.

According to liquid phase growth method, an Si epitaxial layer of 5 μwas grown at a lower temperature on the N-type (100) porous Sisubstrate. The conditions for growth are as follows;

solvent: Sn,

Solute: Si

growth temperature: 900° C.

growth atmosphere: H₂

growth period: 10 minutes

Subsequently, an oxidized layer of 1000 angstroms was formed on thesurface of the epitaxial layer, and another Si substrate on the surfaceof which was formed an oxidized layer of 5000 angstroms was bonded tothe oxidized surface. By heating in oxygen atmosphere at 800° C. for 0.5hour, both of the Si substrates were strongly bonded together.

Then, the bonded substrates were selectively etched with 49%hydrofluoric acid while under stirring. Only the single-crystal Si layerremained without etching in 78 minutes, while the porous Si substratewas selectively etched with the single-crystal Si as a material foretching stopper and then completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 78 minuteslayer, so that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in the non-porous layer (several tens angstroms)is a practically negligible decrease in membrane thickness. That is, theSi substrate of a thickness of 200 μ, rendered porous, was removed, anda single-crystal Si layer of a thickness of 0.5 μm was formed on theSiO₂.

EXAMPLE 68

A P-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in HF solution.

The conditions for anodization are as follows;

applied voltage: 2.6 V

current density: 30 mA·cm⁻²

solution for anodization: HF:H₂O:C₂H₅OH=1:1:1

time: 1.6 hours

thickness of porous Si: 200 μm

porosity: 56%.

According to low pressure CVD method, an Si epitaxial layer of 1.0 μ wasgrown at a lower temperature on the P-type (100) porous Si substrate.The conditions for deposition are as follows;

source gas: SiH₄

carrier gas: H₂

temperature: 850° C.

pressure: 1×10⁻² Torr

growth rate: 3.3 nm/sec.

Subsequently, an oxidized layer of 1000 angstroms was formed on thesurface of the epitaxial layer, and another Si substrate on the surfaceof which was formed an oxidized layer of 5000 angstroms was bonded tothe oxidized surface. By heating in oxygen atmosphere at 800° C. for 0.5hour, both of the Si substrates were strongly bonded together.

Then, the bonded substrates were selectively etched with 49%hydrofluoric acid while under stirring. In 78 minutes, only thesingle-crystal Si layer remained without etching in 78 minutes, whilethe porous Si substrate was selectively etched with the single-crystalSi as a material for etching stopper and then completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 78 minuteslater, so that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in the non-porous layer (several tens angstroms)is a practically negligible decrease in membrane thickness. That is, theSi substrate, rendered porous, of a thickness of 200 μ was removed, anda single-crystal Si layer of a thickness of 1.0 μm was formed on theSiO₂. When SiH₂Cl₂ was used as a source gas, it was required to raisethe growth temperature by several tens of degrees. Nevertheless, theaccelerating etching characteristics to porous substrates wasmaintained.

EXAMPLE 69

According to low pressure CVD method, an Si epitaxial layer of 1 μ wasgrown at a lower temperature on a P-type (100) porous Si substrate. Theconditions for deposition are as follows;

reactive gas flow rate:

SiH₂Cl₂ 1000 SCCM

H₂ 230 1/min

temperature: 1080° C.

pressure: 80 Torr

time: 2 min.

The substrate was anodized in 50% HF solution. The current density thenwas 100 mA/cm². The porous structure formation rate then was 8.4μm/min., and the P-type (100) Si substrate of a thickness of 200 μ wasrendered porous in its entirety for 24 minutes. According to the presentanodization, as has been described above, only the P-type (100) Sisubstrate was rendered porous, but no change was observed in the Siepitaxial layer.

Subsequently, an oxidized layer of 1000 angstroms was formed on thesurface of the epitaxial layer, and another Si substrate on the surfaceof which was formed an oxidized layer of 5000 angstroms was bonded tothe oxidized surface. By heating in oxygen atmosphere at 800° C. for 0.5hour, both of the Si substrates were strongly bonded together.

Then, the bonded substrates were selectively etched with 49%hydrofluoric acid while under stirring. In 78 minutes, only thesingle-crystal Si layer remained without etching, while the porous Sisubstrate was selectively etched with the single-crystal Si as amaterial for etching stopper and then completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 78 minuteslater, so that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in the non-porous layer (several tens angstroms)is a practically negligible decrease in membrane thickness. That is theSi substrate of a thickness of 200 μ, rendered porous, was removed, anda single-crystal Si layer of a thickness of 1.0 μm was formed on theSiO₂.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLE 70

According to atmospheric pressure CVD method, an Si epitaxial layer of 5μ was grown at a lower temperature on a P-type (100) Si substrate. Theconditions for deposition are as follows;

reactive gas flow rate:

SiH₂Cl₂ 1000 SCCM

H₂ 230 1/min

temperature: 1080° C.

pressure: 760 Torr

time: 1 min.

The Si substrate was anodized in HF solution.

The conditions for anodization are as follows;

applied voltage: 2.6 V

current density: 30 mA·cm⁻²

solution for anodization: HF:H₂O:C₂H₅OH=1:1:1

time: 1.6 hours

thickness of porous Si: 200 μm

porosity: 56%.

According to the present anodization as has been described above, onlythe P-type (100) Si substrate was rendered porous, but no change wasobserved in the Si epitaxial layer.

Subsequently, an oxidized layer of 1000 angstroms was formed on thesurface of the epitaxial layer, and another Si substrate on the surfaceof which was formed an oxidized layer of 5000 angstroms was bonded tothe oxidized surface. By heating in oxygen atmosphere at 800 ° C. for0.5 hour, both of the Si substrates were strongly bonded together.

Then, the bonded substrates were selectively etched with 49%hydrofluoric acid while under stirring. In 78 minutes, only thesingle-crystal Si layer remained without etching, while the porous Sisubstrate was selectively etched with the single-crystal Si as amaterial for etching stopper and then completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 78 minuteslater, so that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in the non-porous layer (several tens angstroms)is a practically negligible decrease in membrane thickness. That is, theSi substrate, rendered porous, of a thickness of 200 μ was removed, anda. single-crystal Si layer of a thickness of 5 μm was formed on theSiO₂. As a result of observation of the section under atransmission-type electron microscope, it was confirmed that no newcrystal defect was introduced in the Si layer and that excellentcrystallinity was maintained.

EXAMPLE 71

A N-type Si layer of 1 μ was formed on the surface of a P-type (100) Sisubstrate of a thickness of 200 μ, by ion implantation of proton. Theimplanted amount of H⁺ was 5×10¹⁵ ions/cm². The substrate was anodizedin 50% HF solution. The current density then was 100 mA/cm² The porousstructure formation rate then was 8.4 μm/min., and the P-type (100) Sisubstrate of a thickness of 200 μ was rendered porous in its entiretyfor 24 minutes. According to the present anodization as has beendescribed above, only the P-type (100) Si substrate was rendered porous,but no change was observed in the N-type Si layer.

Subsequently, an oxidized layer of 1000 angstroms was formed on thesurface of the N-type Si layer, and another Si substrate on the surfaceof which was formed an oxidized layer of 5000 angstroms was bonded tothe oxidized surface. By heating in oxygen atmosphere at 800° C. for 0.5hour, both of the Si substrates were strongly bonded together.

Then, the bonded substrates were selectively etched with 49%hydrofluoric acid with stirring. In 78 minutes, only the single-crystalSi layer remained without etching, while the porous Si substrate wasselectively etched with the single-crystal Si as a material for etchingstopper and then completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 78 minuteslater, so that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in the non-porous layer (several tens angstroms)is a practically negligible decrease in membrane thickness. That is, theSi substrate, rendered porous, of a thickness of 200 μ was removed, anda single-crystal Si layer of a thickness of 1.0 μm was formed on SiO₂.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLE 72

A P-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in 50% HF solution. The current density then was 100 mA/cm².The porous structure formation rate then was about 8.4 μm/min., and theP-type (100) Si substrate of a thickness of 200 μ was rendered porous inits entirety for 24 minutes.

According to MBE (monocular beam epitaxy) method, an Si epitaxial layerwas grown at a lower temperature on the P-type (100) porous Sisubstrate. The conditions for deposition are as follows;

temperature: 700° C.

pressure: 1×10⁻⁹ Torr

growth rate: 0.1 nm/sec.

Subsequently, a substrate of fused silica glass processed with opticalpolishing was bonded onto the the surface of the epitaxial layer, andboth of the substrates were strongly bonded together by heating at 800°C. for 0.5 hour in oxygen atmosphere.

According to plasma CVD method, Si₃N₄ was deposited to 0.1 μm, therebycoating the bonded two substrates. Thereafter, only the nitride membraneon the porous substrate was removed by reactive ion etching. Then, thebonded substrates were selectively etching with a mixed solution of 49%hydrofluoric acid and alcohol (10:1), without stirring. In 82 minutes,only the single-crystal Si layer remained without etching, while theporous Si substrate was selectively etched with the single-crystal Si asa material for etching stopper, and completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 82 minuteslater, so that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in the non-porous layer (several tens angstroms)is a practically negligible decrease in membrane thickness. That is, theSi substrate of a thickness of 200 μ, rendered porous, was removed, andafter the removal of the Si₃N₄ layer, a single-crystal Si layer of athickness of 0.5 μm was formed on the substrate of the glass.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLES 73 TO 86

The same procedure as in Examples 42 to 55 was effected, replacing theetching solution used in Examples 42 to 55 with that in Example 72. Inany of the present. Examples, consequently, a single-crystal Si layerwas formed with extremely less crystal defect on insulating materials.

EXAMPLE 87

A N-type. Si layer of 1 μ was formed on the surface of a P-type (100) Sisubstrate of a thickness of 200 μ, by ion implantation of proton. Theimplanted amount of H⁺ was 5×10¹⁵ ions/cm The substrate was anodized in50% HF solution. The current density then was 100 mA/cm². The porousstructure formation rate then was 8.4 μm/min., and the P-type (100) Sisubstrate of a thickness of 200 μ was rendered porous in its entiretyfor 24 minutes. According to the present anodization as has beendescribed above, only the P-type (100) Si substrate was rendered porous,but no change was observed in the N-type Si layer.

Subsequently, a second Si substrate on the surface of which was formedan oxidized layer of 5000 angstroms, was bonded to the surface of theN-type Si layer. By heating in oxygen atmosphere at 800° C. for 0.5hour, both of the Si substrates were strongly bonded together.

Then, the bonded substrates were selectively etched with a mixedsolution of 49% hydrofluoric acid and alcohol (10:1) without stirring.In 82 minutes, only the single-crystal Si layer remained withoutetching, while the porous Si substrate was selectively etched with thesingle-crystal Si as a material for etching stopper and then completelyremoved.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 82 minuteslater, so that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in the non-porous layer (several tens angstroms)is a practically negligible decrease in membrane thickness. That is, theSi substrate of a thickness of 200 μ, rendered porous, was removed, anda single-crystal Si layer of a thickness of 1.0 μm was formed on theSiO₂ layer.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLES 88 TO 102

The same procedure as in Examples 57 to 71 was effected, replacing theetching solution in Examples 57 to 71 with that in Example 72. In any ofthe present Examples, consequently, a single-crystal Si layer was formedwith extremely less crystal defect on insulating materials.

EXAMPLE 103

A P-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in 50% HF solution.

The current density then was 100 mA/cm^(2.) The porous structureformation rate then was about 8.4 μm/min., and the P-type (100) Sisubstrate of a thickness of 200 μ was rendered porous in its entiretyfor 24 minutes.

According to MBE (molecular beam epitaxy) method, an Si epitaxial layerof 0.5 μ was grown at a lower temperature on the P-type (100) porous Sisubstrate. The conditions for deposition are as follows;

temperature: 700° C.

pressure: 1×10⁻⁹ Torr

growth rate: 0.1 nm/sec.

Subsequently, a substrate of fused silica glass processed with opticalpolishing was bonded onto the surface of the epitaxial layer, and bothof the substrates were strongly bonded together by heating at 800° C.for 0.5 hour in oxygen atmosphere.

According to plasma CVD method, Si₃N₄ was deposited at 0.1 μm, therebycoating the bonded two substrates. Thereafter, only the nitride membraneon the porous substrate was removed by reactive ion etching. Then, thebonded substrates were selectively etched with a mixed solution of 49%hydrofluoric acid and aqueous hydrogen peroxide solution (1:5), whileunder stirring. In 62 minutes, only the single-crystal Si layer remainedwithout etching, while the porous Si substrate was selectively etchedwith the single-crystal Si as a material for etching stopper andcompletely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 62 minuteslater, so that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in the non-porous layer (several tens angstroms)is a practically negligible decrease in membrane thickness. That is, theSi substrate of a thickness of 200 μ, rendered porous, was removed, andafter the removal of the Si₃N₄ layer, a single-crystal Si layer of athickness of 0.5 μm was formed on the substrate of the glass.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLES 104 TO 118

The same procedure as in Examples 42 to 56 was effected, replacing theetching solution in Examples 42 to 56 with that in Example 103. In anyof the present Examples, consequently, a single-crystal Si layer wasformed with extremely less crystal defect on insulating materials.

EXAMPLE 119

A P-type (100.) single-crystal Si substrate of a thickness of 200 μ wasanodized in 50% HF solution. The current density then was 100 mA/cm² Theporous structure formation rate then was about 8.4 μm/min., and theP-type (100) Si substrate of a thickness of 200 μ was rendered porous inits entirety for 24 minutes.

According to MBE (molecular beam epitaxy) method, an Si epitaxial layerof 0.5 μ was grown at a lower temperature on the P-type (100) porous Sisubstrate. The conditions for deposition are as follows;

temperature: 700° C.

pressure: 1×10⁻⁹ Torr

growth rate: 0.1 nm/sec.

Subsequently, the surface of the epitaxial layer was thermally oxidizedin a depth of 50 nm. A substrate of fused silica glass processed withoptical polishing was bonded onto the thermally oxidized membrane, andboth of the substrates were strongly bonded together by heating at 800°C. for 0.5 hour in oxygen atmosphere.

According to low pressure CVD method, Si₃N₄ was deposited to 0.1 μm,thereby coating the bonded two substrates. Thereafter, only the nitridemembrane on the porous substrate was removed by reactive ion etching.

Then, the bonded substrates were selectively etched with a mixedsolution of 49% hydrofluoric acid and aqueous hydrogen peroxide solution(1:5). In 62 minutes, only the single-crystal Si layer remained withoutetching, while the porous Si substrate was selectively etched with thesingle-crystal Si as a material for etching stopper and completelyremoved.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as SO angstroms or less even 62 minuteslater, so that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. The etched amount in the non-porous layer (several tens angstroms)is a practically negligible decrease in membrane thickness. That is, theSi substrate of a thickness of 200 μ, rendered porous, was removed, andafter the removal of the Si₃N₄ layer, a single-crystal Si layer of athickness of 0.5 μm was formed on the substrate of the silica glass.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLES 120 TO 133

The same procedure as in Examples 58 to 71 was effected, replacing theetching solution in Examples 58 to 71 with that in Example 103. In anyof the present Examples, consequently, a single-crystal Si layer wasformed with extremely less crystal defect on insulating materials.

EXAMPLE 134

A P-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in 50% HF solution. The current density then was 100 mA/cm².The porous structure formation rate then was 8.4 μm/min., and the P-type(100) Si substrate of a thickness of 200 μ was rendered porous in itsentirety for 24 minutes.

According to MBE (molecular beam epitaxy) method, an Si epitaxial layerof 0.5 μ was grown at a lower temperature on the P-type (100) porous Sisubstrate. The conditions for deposition are as follows;

temperature: 700° C.

pressure: 1×10⁻⁹ Torr

growth rate: 0.1 nm/sec.

Subsequently, a substrate of fused silica glass processed with opticalpolishing was bonded onto the surface of the epitaxial layer, and bothof the substrates were strongly bonded together by heating at 800° C.for 0.5 hour in oxygen atmosphere.

According to plasma CVD method, Si₃N₄ was deposited to 0.1 μm, therebycoating the bonded two substrates. Thereafter, only the nitride membraneon the porous substrate was removed by reactive ion etching. Then, thebonded substrates were selectively etched with a mixed solution of 49%hydrofluoric acid, alcohol and aqueous hydrogen peroxide solution(10:6:50), without stirring. In 65 minutes, only the single-crystal Silayer remained without etching, while the porous Si substrate wasselectively etched with the single-crystal Si as a material for etchingstopper and completely removed.

The etching rate of the non-porous Si single crystal with the etchingsolution was extremely low, such as approximately slightly less than 40angstroms even 65 minutes later, so that the selective ratio of theetching rate of the porous layer to that of the non-porous Sisingle-crystal was as large as 10⁵ or more. The etched amount in thenon-porous layer (several tens angstroms) is a practically negligibledecrease in membrane thickness. That is, the Si substrate of a thicknessof 200 μ, rendered porous, was removed, and after the removal of theSi₃N₄ layer, a single-crystal Si layer of a thickness of 0.5 μm wasformed on the substrate of the silica glass.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLES 135 TO 148

The same procedure as in Examples 42 to 55 was effected, replacing theetching solution in Examples 42 to 55 with that in Example 134. In anyof the present Examples, consequently, a single-crystal Si layer wasformed with extremely less crystal defect on insulating materials.

EXAMPLE 149

A N-type Si layer of 1 μ was formed on the surface of a P-type (100) Sisubstrate of a thickness of 200 μ, by ion implantation of proton. Theimplanted amount of H⁺ was 5×10¹⁵ ions/cm The substrate was anodized in50% HF solution. The current density then was 100 mA/cm^(2.) The porousstructure formation rate then was 8.4 μm/min., and the P-type (100) Sisubstrate of a thickness of 200 μ was rendered porous in its entiretyfor 24 minutes. According to the present anodization as has beendescribed above, only the P-type (100) Si substrate was rendered porous,but no change was observed in the N-type Si layer.

Subsequently, a second Si substrate on the surface of which was formedan oxidized layer of 5000 angstroms was bonded to the surface of theN-type Si layer. By heating in oxygen atmosphere at 800° C. for 0.5hour, both of the substrates were strongly bonded together.

Then, the bonded substrates were selectively etched with a mixedsolution of 49% hydrofluoric acid, alcohol and aqueous 30% hydrogenperoxide solution (10:6:50) without stirring. In 65 minutes, only thesingle-crystal Si layer remained without etching, while the porous Sisubstrate was selectively etched with the single-crystal Si as amaterial for etching stopper, and then completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 65 minuteslater, so that the selective ratio of the etching rate of the porouslayer to that of the non-porous Si single-crystal was as large as 10⁵ ormore. T he etched amount in the non-porous layer (several tensangstroms) is a practically negligible decrease in membrane thickness.That is, the Si substrate of a thickness of 200 μ, rendered porous, wasremoved, and a single-crystal Si layer of a thickness of 1.0 μm wasformed on the SiO₂ layer.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLES 150 TO 164

The same procedure as in Examples 57 to 71 was effected, replacing theetching solution in Examples 57 to 71 with that in Example 134. In anyof the present Examples, consequently, a single-crystal Si layer wasformed with extremely less crystal defect on insulating materials.

EXAMPLE 165

A P-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in 50% HF solution. The current density then was 100 mA/cm².The porous structure formation rate then was 6.4 μm/min., and the P-type(100) Si substrate of a thickness of 200 μ was rendered porous in itsentirety for 24 minutes.

According to MBE (molecular beam epitaxy) method, an Si epitaxial layerof 0.5 μ was grown at a lower temperature on the P-type (100) porous Sisubstrate. The conditions for deposition are as follows;

temperature: 700° C.

pressure: 1×10⁻⁹ Torr

growth rate: 0.1 nm/sec.

Subsequently, a substrate of fused silica glass processed with opticalpolishing was bonded onto the surface of the epitaxial layer, and bothof the substrates were strongly bonded together by heating at 800° C.for 0.5 hour in oxygen atmosphere.

According to plasma CVD method, Si₃N₄ was deposited to 0.1 μm, therebycoating the bonded two substrates. Thereafter, only the nitride membraneon the porous substrate was removed by reactive ion etching. Then, thebonded substrates were selectively etched, under stirring, with bufferedhydrofluoric acid. In 258 minutes, only the single-crystal Si layerremained without etching, while the porous Si substrate was selectivelyetched with the single-crystal Si as a material for etching stopper andthen completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 100 angstroms or less even 258minutes later, so that the selective ratio of the etching rate of theporous layer to that of the non-porous Si single-crystal was as large as10⁵ or more. The etched amount in the non-porous layer (several tensangstroms) is a practically negligible decrease in membrane thickness.That is, the Si substrate of a thickness of 200 μ, rendered porous, wasremoved, and after the removal of the Si N layer, a single-crystal Silayer of a thickness of 0.5 μm was formed on the substrate of the glass.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLES 166 TO 180

The same procedure as in Examples 42 to 56 was effected, replacing theetching solution in Examples 42 to 56 with that in Example 165. In anyof the present Examples, consequently, a single-crystal Si layer wasformed with extremely less crystal defect on insulating materials.

EXAMPLE 181

A P-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in 50% HF solution. The current density then was 100 mA/cm³.The porous structure formation rate then was about 8.4 μm/min., and theP-type (100) Si substrate of a thickness of 200 μ was rendered porous inits entirety for 24 minutes.

According to MBE (molecular beam epitaxy) method, an Si epitaxial layerof 0.5 μ was grown at a lower temperature on the P-type (100) porous Sisubstrate. The conditions for deposition are as follows;

temperature: 700° C.

pressure: 1×10⁻⁹ Torr

growth rate: 0.1 nm/sec.

Subsequently, the surface of the epitaxial layer was thermally oxidizedin a depth of 50 nm. A substrate of fused silica glass processed withoptical polishing was bonded onto the thermally oxidized membrane, andboth of the substrates were strongly bonded together by heating at 800°C. for 0.5 hour in oxygen atmosphere.

According to low pressure CVD method, Si₃N₄ was deposited to 0.1 μm,thereby coating the bonded two substrates. Thereafter, only the nitridemembrane on the porous substrate was removed by reactive ion etching.

Then, the bonded substrates were immersed in buffered hydrofluoric acid,and stirred. In 258 minutes, only the single-crystal Si layer remainedwithout etching, while the porous Si substrates was selectively etchedwith the single-crystal Si as a material for etching stopper and thencompletely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 100 angstroms or less even 258minutes later, so that the selective ratio of the etching rate of theporous layer to that of the non-porous Si single-crystal was as large as10⁵ or more. The etched amount in the non-porous layer (several tensangstroms) is a practically negligible decrease in membrane thickness.That is, the Si substrate of a thickness of 200 μ, rendered porous, wasremoved, and after the removal of the Si₃N₄ layer, a single-crystal Silayer of a thickness of 0.5 μm on the substrate of the silica glass wasformed.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLES 182 TO 195

The same procedure as in Examples 58 to 71 was effected, replacing theetching solution in Examples 58 to 71 with that in Example 165. In anyof the present Examples, consequently, a single-crystal Si layer wasformed with extremely less crystal defect on insulating materials.

EXAMPLE 196

A P-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in 50% HF solution. The current density then was 100 mA/cm².The porous structure formation rate then was 8.4 μm/min., and the P-type(100) Si substrate of a thickness of 200 μ was rendered porous in itsentirety for 24 minutes.

According to MBE (molecular beam epitaxy) method, an Si epitaxial layerof 0.5 μ was grown at a lower temperature on the P-type (100) porous Sisubstrate. The conditions for deposition are as follows;

temperature: 700° C.

pressure: 1×10⁻⁹ Torr

growth rate: 0.1 nm/sec.

Subsequently, a substrate of fused silica glass processed with opticalpolishing was bonded onto the surface of the epitaxial layer, and bothof the substrates were strongly bonded together by heating at 800° C.for 0.5 hour in oxygen atmosphere.

According to plasma CVD method, Si₃N₄ was deposited to 0.1 μm, therebycoating the bonded two substrates. Thereafter, only the nitride membraneon the porous substrate was removed by reactive ion etching. Then, thebonded substrates were selectively etched in a mixed solution ofbuffered hydrofluoric acid and alcohol (10:1) without stirring. In 275minutes, only the single-crystal Si layer remained without etching,while the porous Si substrate was selectively etched with thesingle-crystal Si as a material for etching stopper and completelyremoved.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 100 angstroms or less even 275minutes later, so that the selective ratio of the etching rate of theporous layer to that of the non-porous Si single-crystal was as large as10⁵ or more. The etched amount in the non-porous layer (several tensangstroms) is a practically negligible decrease in membrane thickness.That is, the Si substrate of a thickness of 200 μ, rendered porous, wasremoved, and after the removal of the Si₃N₄ layer, a single-crystal Silayer of a thickness of 0.5 μm was formed on the substrate of the glass.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLES 197 TO 210

The same procedure as in Examples 42 to 55 was effected, replacing theetching solution in Examples 42 to 55 with that in Example 196. In anyof the present Examples, consequently, a single-crystal Si layer wasformed with extremely less crystal defect on insulating materials.

EXAMPLE 211

A N-type Si layer of thickness of 1 μ was formed on the surface of aP-type Si layer of 200 μ in thickness, by ion implantation of proton.The implanted amount of H⁺ was 5×10¹⁵ ions/cm².

The substrate was anodized in 50% HF solution. The current density thenwas 100 mA/cm². The porous structure formation rate then was 8.4μm/min., and the N-type (100) Si substrate of a thickness of 200 μ wasrendered porous in its entirety for 24 minutes. According to the presentanodization as has been described above, only the P-type (100) Sisubstrate was rendered porous, but no change was observed in the N-typeSi layer.

Subsequently, a second Si substrate on the surface of which was formedan oxidized layer of 5000 angstroms was bonded to the surface of theN-type Si layer. By heating in oxygen atmosphere at 800° C. for 0.5hour, both of the substrates were strongly bonded together.

Then, the bonded substrates were selectively etched with a mixedsolution of buffered hydrofluoric acid (HF:4.46%, NH₄F:36.2%) and ethylalcohol (10:1), without stirring. In 275 minutes, only thesingle-crystal Si layer remained without etching, while the porous Sisubstrate was selectively etched with the single-crystal Si as amaterial for etching stopper and then completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as approximately slightly less than 40angstroms even 275 minutes later, so that the selective ratio of theetching rate of the porous layer to that of the non-porous Sisingle-crystal was as large as 10⁵ or more. The etched amount in thenon-porous layer (several tens angstroms) is a practically negligibledecrease in membrane thickness. That is, the Si substrate of a thicknessof 200 μ, rendered porous, was removed, and a single-crystal Si layer ofa thickness of 1.0 μm was formed on the SiO₂ layer.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLES 212 TO 226

The same procedure as in Examples 57 to 71 was effected, replacing theetching solution in Examples 57 to 71 with that in Example 196. In anyof the present Examples, consequently, a single-crystal Si layer wasformed with extremely less crystal defect on insulating materials.

EXAMPLE 227

A P-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in 50% HF solution. The current density then was 100 mA/cm².The porous structure formation rate then was 8.4 μm/min., and the P-type(100) Si substrate of a thickness of 200 μ was rendered porous in itsentirety for 24 minutes.

According to MBE (molecular beam epitaxy) method, an Si epitaxial layerof 0.5 μ was grown at a lower temperature on the P-type (100) porous Sisubstrate. The conditions for deposition are as follows;

temperature: 700° C.

pressure: 1×10⁻⁹ Torr

growth rate: 0.1 nm/sec.

Subsequently, a substrate of fused silica glass processed with opticalpolishing was bonded onto the surface of the epitaxial layer, and bothof the substrates were strongly bonded together by heating at 800° C.for 0.5 hour in oxygen atmosphere.

According to plasma CVD method, Si₃N₄ was deposited to 0.1 μm, therebycoating the bonded two substrates. Thereafter, only the nitride membraneon the porous substrate was removed by reactive ion etching. Then, thebonded substrates were selectively etched in a mixed solution ofbuffered hydrofluoric acid and aqueous hydrogen peroxide solution (1:5)with stirring. In 190 minutes, only the single-crystal Si layer remainedwithout etching, while the porous Si substrate was selectively etchedwith the single-crystal Si as a material for etching stopper, andcompletely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 50 angstroms or less even 190minutes later, so that the selective ratio of the etching rate of theporous layer to that of the non-porous Si single-crystal was as large as10⁵ or more. The etched amount in the non-porous layer (several tensangstroms) is a practically negligible decrease in membrane thickness.That is, the Si substrate of a thickness of 200 μ, rendered porous, wasremoved, and after the removal of the Si₃N₄ layer, a single-crystal Silayer of a thickness of 0.5 μm was formed on the substrate of the silicaglass.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLES 228 TO 242

The same procedure as in Examples 42 to 56 was effected, replacing theetching solution in Examples 42 to 56 with that in Example 227. In anyof the present Examples, consequently, a single-crystal Si layer wasformed with extremely less crystal defect on insulating materials.

EXAMPLE 243

A P-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in 50% HF solution.

The current density then was 100 mA/cm². The porous structure formationrate then was about 8.4 μm/min., and the P-type (100) Si substrate of athickness of 200 μ was rendered porous in its entirety for 24 minutes.

According to MBE (molecular beam epitaxy) method, an Si epitaxial layerof 0.5 μ was grown at a lower temperature on the P-type (100) porous Sisubstrate. The conditions for deposition are as follows;

temperature: 700° C.

pressure: 1×10⁻⁹ Torr

growth rate: 0.1 nm/sec.

subsequently, the surface of the epitaxial layer was thermally oxidizedin a depth of 50 nm. A substrate of fused silica glass processed withoptical polishing was bonded onto the thermally oxidized membrane, andboth of the substrates were strongly bonded together by heating at 800°C. for 0.5 hour in oxygen atmosphere.

According to low pressure CVD method, Si₃N₄ as deposited to 0.1 μm,thereby coating the bonded two substrates. Thereafter, only the nitridemembrane on the porous substrate was removed by reactive ion etching.

Then, the bonded substrates were immersed in a mixed solution ofbuffered hydrofluoric acid and aqueous hydrogen peroxide solution (1:5),and stirred. In 190 minutes, only the single-crystal Si layer remainedwithout etching, while the porous Si substrate was selectively etchedwith the single-crystal Si as a material for etching stopper and thencompletely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as 70 angstroms or less even 190minutes later, so that the selective ratio of the etching rate of theporous layer to that of the non-porous Si single-crystal was as large as10⁵ or more. The etched amount in the non-porous layer (several tensangstroms) is a practically negligible decrease in membrane thickness.That is, the Si substrate of a thickness of 200 μ, rendered porous, wasremoved, and after the removal of the Si₃N₄ layer, a single-crystal Silayer of a thickness of 0.5 μm was formed on the substrate of the silicaglass.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLES 244 TO 257

The same procedure as in Examples 58 to 71 was effected, replacing theetching solution in Examples 58 to 71 with that in Example 243. In anyof the present Examples, consequently, a single-crystal Si layer wasformed with extremely less crystal defect on insulating materials.

EXAMPLE 258

A P-type (100) single-crystal Si substrate of a thickness of 200 μ wasanodized in 50% HF solution. The current density then was 100 mA/cm².The porous structure formation rate then was 8.4 μm/min., and the P-type(100) Si substrate of a thickness of 200 μ was rendered porous in itsentirety for 24 minutes.

According to MBE (molecular beam epitaxy) method, an Si epitaxial layerof 0.5 μ was grown at a lower temperature on the P-type (100) porous Sisubstrate. The conditions for deposition are as follows;

temperature: 700° C.

pressure: 1×10⁻⁹ Torr

growth rate: 0.1 nm/sec.

Subsequently, a substrate of fused silica lass processed with opticalpolishing was bonded onto he surface of the epitaxial layer, and both ofthe substrates were strongly bonded together by heating at 800° C. for0.5 hour in oxygen atmosphere.

According to plasma CVD method, Si₃N₄ was deposited to 0.1 μm, therebycoating the bonded two substrates. Thereafter, only the nitride membraneon the porous substrate was removed by reactive ion etching. Then, thebonded substrates were selectively etched in a mixed solution ofbuffered hydrofluoric acid, alcohol and aqueous hydrogen peroxidesolution (10:6:50) without stirring. In 205 minutes, only thesingle-crystal Si layer remained without etching, while the porous Sisubstrate was selectively etched with the single-crystal Si as amaterial for etching stopper and then completely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as approximately slightly less than 40angstroms even 205 minutes later, so that the selective ratio of theetching rate of the porous layer to that of the non-porous Sisingle-crystal was as large as 10⁵ or more. The etched amount in thenon-porous layer (several tens angstroms) is a practically negligibledecrease in membrane thickness. That is, the Si substrate of a thicknessof 200 μ, rendered porous, was removed, and after the removal of theSi₃N₄ layer, a single-crystal Si layer of a thickness of 0.5 μm wasformed on the substrate of the silica glass.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLES 259 TO 272

The same procedure as in Examples 42 to 55 was effected, replacing theetching solution in Examples 42 to 55 with that in Example 258. In anyof the present Examples, consequently, a single-crystal Si layer wasformed with extremely less crystal defect on insulating materials.

EXAMPLE 273

A N-type Si layer of thickness of 1 μ was formed on the surface of aP-type (100) Si substrate of thickness of 200 μ, by ion implantation ofproton. The implanted amount of H⁺ was 5×10⁻¹⁵ ions/cm².

The substrate was anodized in 50% HF solution. The current density thenwas 100 mA/cm². The porous structure formation rate then was 8.4μm/min., and the P-type (100) Si substrate of a thickness of 200 μ wasrendered porous in its entirety for 24 minutes. According to the presentanodization as has been described above, only the P-type (100) Sisubstrate was rendered porous, but no change was observed in the N-typeSi layer.

Subsequently, a second Si substrate on the surface of which was formedan oxidized layer of 5000 angstroms was bonded to the surface of theN-type Si layer. By heating in oxygen atmosphere at 800° C. for 0.5hour, both of the substrates were strongly bonded together.

Then, the bonded substrates were selectively etched with a mixedsolution of buffered hydrofluoric acid (HF:4.46%, NH₄F:36.2%), ethylalcohol and 30% aqueous hydrogen peroxide solution (10:6:50), withoutstirring. In 180 minutes, only the single-crystal Si layer remainedwithout etching, while the porous Si substrate was selectively etchedwith the single-crystal Si as a material for etching stopper and thencompletely removed.

The etching rate of the non-porous Si single-crystal with the etchingsolution was extremely low, such as approximately slightly less than 40angstroms even 180 minutes later, so that the selective ratio of theetching rate of the porous layer to that of the non-porous Sisingle-crystal was as large as 10⁵ or more. The etched amount in thenon-porous layer (several tens angstroms) is a practically negligibledecrease in membrane thickness. That is, the Si substrate of a thicknessof 200 μ, rendered porous, was removed, and a single-crystal Si layer ofa thickness of 1.0 μm was formed on the SiO₂ layer.

As a result of observation of the section under a transmission-typeelectron microscope, it was confirmed that no new crystal defect wasintroduced in the Si layer and that excellent crystallinity wasmaintained.

EXAMPLES 274 TO 288

The same procedure as in Examples 57 to 71 was effected, replacing theetching solution in Examples 57 to 71 with that in Example 273. In anyof the present Examples, consequently, a single-crystal Si layer wasformed with extremely less crystal defect on insulating materials.

What is claimed is:
 1. An etching method comprising the steps of:providing a substrate having a porous silicon layer and a non-poroussilicon layer; and selectively etching the porous silicon layer from thesubstrate with an etching liquid comprising hydrogen fluoride, hydrogenperoxide and alcohol to leave the non-porous silicon layer.
 2. Theetching method according to claim 1, wherein the etching liquidcomprises a solution comprising 1% to 95% of hydrogen peroxide and up to40% of an alcohol.
 3. The etching method according to claim 1, whereinsaid step of selectively etching the porous silicon layer is carried outat a temperature in the range of 0° C. to 100° C.
 4. The etching methodaccording to claim 3, wherein said step of selectively etching theporous silicon layer is carried out at a temperature in the range of 5°C. to 80° C.
 5. The etching method according to claim 4, wherein saidstep of selectively etching the porous silicon layer is carried out at atemperature in the range of 5° C. to 60° C.
 6. A method for preparing asemiconductor member comprising the steps of: providing a firstsubstrate having a porous monocrystalline silicon layer and a non-porousmonocrystalline silicon layer; bonding the first substrate to a secondsubstrate with an insulating layer therebetween such that the non-porousmonocrystalline silicon layer forms an interior layer of a multi-layerstructure; and selectively etching and removing the porousmonocrystalline silicon layer from the multi-layer structure with anetching liquid comprising hydrogen fluoride, hydrogen peroxide andalcohol, to leave the non-porous silicon layer.
 7. A method forpreparing a semiconductor member comprising the steps of: providing afirst substrate having a porous monocrystalline silicon layer and anon-porous monocrystalline silicon layer; bonding the first substrate toa second light transmissive substrate such that the non-porousmonocrystalline silicon layer forms an interior layer of a multi-layerstructure; and selectively etching and removing the porousmonocrystalline silicon layer from the multi-layer structure with anetching liquid comprising hydrogen fluoride, hydrogen peroxide andalcohol, to leave the non-porous silicon layer.
 8. The method accordingto claim 6 or 7, wherein the etching liquid comprises 1% to 95% ofhydrogen peroxide and at most 40% of alcohol.
 9. The method according toclaim 6 or 7, wherein said step of selectively etching the porousmonocrystalline silicon layer is carried out at a temperature in therange of 0° C. to 100° C.
 10. The method according to claim 9, whereinsaid step of selectively etching the porous monocrystalline siliconlayer is carried out at a temperature in the range of 5° C. to 80° C.11. The method according to claim 10, wherein said step of selectivelyetching the porous monocrystalline silicon layer is carried out at atemperature in the range of 5° C. to 60° C.
 12. The method according toclaim 6 or 7, wherein said step of providing a first substrate comprisesmaking a silicon substrate porous to form a porous monocrystallinesilicon layer and epitaxially growing a non-porous monocrystallinesilicon layer on the porous monocrystalline silicon layer.
 13. Themethod according to claim 12, wherein the step of making the siliconsubstrate porous comprises anodizing the silicon substrate.
 14. Themethod according to claim 12, wherein the step of epitaxially growingthe non-porous monocrystalline silicon layer comprises molecular beamepitaxy, plasma chemical vapor deposition, low pressure chemical vapordeposition, atmospheric pressure chemical vapor deposition, liquid phasegrowth or bias sputtering.
 15. The method according to claim 6 or 7,wherein said step of providing a first substrate comprises treating asurface of a silicon substrate to form a porous monocrystalline siliconlayer, wherein an untreated portion of said silicon substrate forms thenon-porous monocrystalline silicon layer.
 16. The method according toclaim 14, wherein said step of providing a first substrate comprisesirradiating p-type silicon with protons to form a n-type silicon layerand anodizing a portion of said p-type silicon to form said porouslayer.
 17. The method according to claim 14, wherein said step ofproviding a first substrate comprises epitaxially growing an intrinsicmonocrystalline silicon layer on a p-type silicon substrate andselectively making portions of said p-type silicon substrate porous toform said porous layer.
 18. The method according to claim 6, wherein thesecond substrate comprises silicon.
 19. The method according to claim 6,wherein the second substrate and the insulating layer are formed byoxidizing a surface of a silicon substrate.
 20. The method according toclaim 6, wherein the first substrate and the insulating layer are formedby oxidizing a surface of the non-porous monocrystalline silicon layerof the first substrate.
 21. The method according to claim 6, wherein theinsulating layer comprises a first insulating layer and a secondinsulating layer, and wherein the first substrate and the firstinsulating layer are formed by oxidizing a surface of the non-porousmonocrystalline silicon layer of the first substrate, and wherein thesecond substrate and the second insulating layer are formed by oxidizinga surface of a silicon substrate.
 22. The method according to claim 7,wherein the second substrate comprises glass.
 23. The method accordingto claim 7, wherein the second substrate comprises quartz.
 24. Anetching method comprising the steps of: providing a substrate having aporous silicon layer and a non-porous silicon layer; and selectivelyetching the porous silicon layer from the substrate with an etchingsolution comprising hydrofluoric acid, hydrogen peroxide and alcohol, toleave the non-porous silicon layer.
 25. A method for preparing asemiconductor member which comprises the steps of: providing a firstsubstrate having a porous monocrystalline silicon layer and a non-porousmonocrystalline silicon layer; bonding the first substrate to a secondsubstrate such that the non-porous monocrystalline silicon layer formsan interior layer of a multi-layer structure; and selectively etchingand removing the porous monocrystalline silicon layer from themulti-layer structure with an etching solution comprising hydrofluoricacid, hydrogen peroxide and alcohol, to leave the non-porous siliconlayer.